[v2,1/4] vmalloc: New flags for safe vfree on special perms

Commit Message

Rick Edgecombe Dec. 12, 2018, 12:03 a.m. UTC

This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
enabling vfree operations to immediately clear executable TLB entries to freed
pages, and handle freeing memory with special permissions.

In order to support vfree being called on memory that might be RO, the vfree
deferred list node is moved to a kmalloc allocated struct, from where it is
today, reusing the allocation being freed.

arch_vunmap is a new __weak function that implements the actual unmapping and
resetting of the direct map permissions. It can be overridden by more efficient
architecture specific implementations.

For the default implementation, it uses architecture agnostic methods which are
equivalent to what most usages do before calling vfree. So now it is just
centralized here.

This implementation derives from two sketches from Dave Hansen and Andy
Lutomirski.

Suggested-by: Dave Hansen <[e-mail]>
Suggested-by: Andy Lutomirski <[e-mail]>
Suggested-by: Will Deacon <[e-mail]>
Signed-off-by: Rick Edgecombe <[e-mail]>
---
 include/linux/vmalloc.h |  2 ++
 mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
 2 files changed, 69 insertions(+), 6 deletions(-)

Comments

Andy Lutomirski Dec. 12, 2018, 2:20 a.m. UTC | #1

On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
<[e-mail]> wrote:
>
> This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
> enabling vfree operations to immediately clear executable TLB entries to freed
> pages, and handle freeing memory with special permissions.
>
> In order to support vfree being called on memory that might be RO, the vfree
> deferred list node is moved to a kmalloc allocated struct, from where it is
> today, reusing the allocation being freed.
>
> arch_vunmap is a new __weak function that implements the actual unmapping and
> resetting of the direct map permissions. It can be overridden by more efficient
> architecture specific implementations.
>
> For the default implementation, it uses architecture agnostic methods which are
> equivalent to what most usages do before calling vfree. So now it is just
> centralized here.
>
> This implementation derives from two sketches from Dave Hansen and Andy
> Lutomirski.
>
> Suggested-by: Dave Hansen <[e-mail]>
> Suggested-by: Andy Lutomirski <[e-mail]>
> Suggested-by: Will Deacon <[e-mail]>
> Signed-off-by: Rick Edgecombe <[e-mail]>
> ---
>  include/linux/vmalloc.h |  2 ++
>  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
>  2 files changed, 69 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 398e9c95cd61..872bcde17aca 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h */
>  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not fully initialized */
>  #define VM_NO_GUARD            0x00000040      /* don't add guard page */
>  #define VM_KASAN               0x00000080      /* has allocated kasan shadow memory */
> +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before releasing pages */
> +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with special perms */
>  /* bits [20..32] reserved for arch specific ioremap internals */
>
>  /*
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 97d4b25d0373..02b284d2245a 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -18,6 +18,7 @@
>  #include <linux/interrupt.h>
>  #include <linux/proc_fs.h>
>  #include <linux/seq_file.h>
> +#include <linux/set_memory.h>
>  #include <linux/debugobjects.h>
>  #include <linux/kallsyms.h>
>  #include <linux/list.h>
> @@ -38,6 +39,11 @@
>
>  #include "internal.h"
>
> +struct vfree_work {
> +       struct llist_node node;
> +       void *addr;
> +};
> +
>  struct vfree_deferred {
>         struct llist_head list;
>         struct work_struct wq;
> @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
>  {
>         struct vfree_deferred *p = container_of(w, struct vfree_deferred, wq);
>         struct llist_node *t, *llnode;
> +       struct vfree_work *cur;
>
> -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> -               __vunmap((void *)llnode, 1);
> +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> +               cur = container_of(llnode, struct vfree_work, node);
> +               __vunmap(cur->addr, 1);
> +               kfree(cur);
> +       }
>  }
>
>  /*** Page table manipulation functions ***/
> @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void *addr)
>         return NULL;
>  }
>
> +/*
> + * This function handles unmapping and resetting the direct map as efficiently
> + * as it can with cross arch functions. The three categories of architectures
> + * are:
> + *   1. Architectures with no set_memory implementations and no direct map
> + *      permissions.
> + *   2. Architectures with set_memory implementations but no direct map
> + *      permissions
> + *   3. Architectures with set_memory implementations and direct map permissions
> + */
> +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)

My general preference is to avoid __weak functions -- they don't
optimize well.  Instead, I prefer either:

#ifndef arch_vunmap
void arch_vunmap(...);
#endif

or

#ifdef CONFIG_HAVE_ARCH_VUNMAP
...
#endif


> +{
> +       unsigned long addr = (unsigned long)area->addr;
> +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> +
> +       /*
> +        * In case of 2 and 3, use this general way of resetting the permissions
> +        * on the directmap. Do NX before RW, in case of X, so there is no W^X
> +        * violation window.
> +        *
> +        * For case 1 these will be noops.
> +        */
> +       if (immediate)
> +               set_memory_nx(addr, area->nr_pages);
> +       if (deallocate_pages && special)
> +               set_memory_rw(addr, area->nr_pages);

Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
want that alias gone before any deallocation happens".
VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
me, please".  deallocate means "this was vfree -- please free the
pages".  I'm not convinced that all the various combinations make
sense.  Do we really need both flags?

(VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
not immediate.)

If we do keep both flags, maybe some restructuring would make sense,
like this, perhaps.  Sorry about horrible whitespace damage.

if (special) {
  /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
  WARN_ON_ONCE(!deallocate_pages);

  if (immediate) {
    /* It's possible that the vmap alias is X and we're about to make
the direct map RW.  To avoid a window where executable memory is
writable, first mark the vmap alias NX.  This is silly, since we're
about to *unmap* it, but this is the best we can do if all we have to
work with is the set_memory_abc() APIs.  Architectures should override
this whole function to get better behavior. */
    set_memory_nx(...);
  }

  set_memory_rw(addr, area->nr_pages);
}


> +
> +       /* Always actually remove the area */
> +       remove_vm_area(area->addr);
> +
> +       /*
> +        * Need to flush the TLB before freeing pages in the case of this flag.
> +        * As long as that's happening, unmap aliases.
> +        *
> +        * For 2 and 3, this will not be needed because of the set_memory_nx
> +        * above, because the stale TLBs will be NX.

I'm not sure I agree with this comment.  If the caller asked for an
immediate unmap, we should give an immediate unmap.  But I'm still not
sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.

> +        */
> +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> +               vm_unmap_aliases();
> +}
> +
>  static void __vunmap(const void *addr, int deallocate_pages)
>  {
>         struct vm_struct *area;
> @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int deallocate_pages)
>         debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
>         debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
>
> -       remove_vm_area(addr);
> +       arch_vunmap(area, deallocate_pages);
> +
>         if (deallocate_pages) {
>                 int i;
>
> @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const void *addr)
>          * nother cpu's list.  schedule_work() should be fine with this too.
>          */
>         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work), GFP_ATOMIC);
> +
> +       /* If no memory for the deferred list node, give up */
> +       if (!w)
> +               return;

That's nasty.  I see what you're trying to do here, but I think you're
solving a problem that doesn't need solving quite so urgently.  How
about dropping this part and replacing it with a comment like "NB:
this writes a word to a potentially executable address.  It would be
nice if we could avoid doing this."  And maybe a future patch could
more robustly avoid it without risking memory leaks.

Rick Edgecombe Dec. 12, 2018, 7:50 p.m. UTC | #2

On Tue, 2018-12-11 at 18:20 -0800, Andy Lutomirski wrote:
> On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> <[e-mail]> wrote:
> > 
> > This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
> > enabling vfree operations to immediately clear executable TLB entries to
> > freed
> > pages, and handle freeing memory with special permissions.
> > 
> > In order to support vfree being called on memory that might be RO, the vfree
> > deferred list node is moved to a kmalloc allocated struct, from where it is
> > today, reusing the allocation being freed.
> > 
> > arch_vunmap is a new __weak function that implements the actual unmapping
> > and
> > resetting of the direct map permissions. It can be overridden by more
> > efficient
> > architecture specific implementations.
> > 
> > For the default implementation, it uses architecture agnostic methods which
> > are
> > equivalent to what most usages do before calling vfree. So now it is just
> > centralized here.
> > 
> > This implementation derives from two sketches from Dave Hansen and Andy
> > Lutomirski.
> > 
> > Suggested-by: Dave Hansen <[e-mail]>
> > Suggested-by: Andy Lutomirski <[e-mail]>
> > Suggested-by: Will Deacon <[e-mail]>
> > Signed-off-by: Rick Edgecombe <[e-mail]>
> > ---
> >  include/linux/vmalloc.h |  2 ++
> >  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
> >  2 files changed, 69 insertions(+), 6 deletions(-)
> > 
> > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > index 398e9c95cd61..872bcde17aca 100644
> > --- a/include/linux/vmalloc.h
> > +++ b/include/linux/vmalloc.h
> > @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h */
> >  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not fully
> > initialized */
> >  #define VM_NO_GUARD            0x00000040      /* don't add guard page */
> >  #define VM_KASAN               0x00000080      /* has allocated kasan
> > shadow memory */
> > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before releasing
> > pages */
> > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with special
> > perms */
> >  /* bits [20..32] reserved for arch specific ioremap internals */
> > 
> >  /*
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 97d4b25d0373..02b284d2245a 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/interrupt.h>
> >  #include <linux/proc_fs.h>
> >  #include <linux/seq_file.h>
> > +#include <linux/set_memory.h>
> >  #include <linux/debugobjects.h>
> >  #include <linux/kallsyms.h>
> >  #include <linux/list.h>
> > @@ -38,6 +39,11 @@
> > 
> >  #include "internal.h"
> > 
> > +struct vfree_work {
> > +       struct llist_node node;
> > +       void *addr;
> > +};
> > +
> >  struct vfree_deferred {
> >         struct llist_head list;
> >         struct work_struct wq;
> > @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> >  {
> >         struct vfree_deferred *p = container_of(w, struct vfree_deferred,
> > wq);
> >         struct llist_node *t, *llnode;
> > +       struct vfree_work *cur;
> > 
> > -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> > -               __vunmap((void *)llnode, 1);
> > +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> > +               cur = container_of(llnode, struct vfree_work, node);
> > +               __vunmap(cur->addr, 1);
> > +               kfree(cur);
> > +       }
> >  }
> > 
> >  /*** Page table manipulation functions ***/
> > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void *addr)
> >         return NULL;
> >  }
> > 
> > +/*
> > + * This function handles unmapping and resetting the direct map as
> > efficiently
> > + * as it can with cross arch functions. The three categories of
> > architectures
> > + * are:
> > + *   1. Architectures with no set_memory implementations and no direct map
> > + *      permissions.
> > + *   2. Architectures with set_memory implementations but no direct map
> > + *      permissions
> > + *   3. Architectures with set_memory implementations and direct map
> > permissions
> > + */
> > +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
> 
> My general preference is to avoid __weak functions -- they don't
> optimize well.  Instead, I prefer either:
> 
> #ifndef arch_vunmap
> void arch_vunmap(...);
> #endif
> 
> or
> 
> #ifdef CONFIG_HAVE_ARCH_VUNMAP
> ...
> #endif
Ok.
> 
> > +{
> > +       unsigned long addr = (unsigned long)area->addr;
> > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > +
> > +       /*
> > +        * In case of 2 and 3, use this general way of resetting the
> > permissions
> > +        * on the directmap. Do NX before RW, in case of X, so there is no
> > W^X
> > +        * violation window.
> > +        *
> > +        * For case 1 these will be noops.
> > +        */
> > +       if (immediate)
> > +               set_memory_nx(addr, area->nr_pages);
> > +       if (deallocate_pages && special)
> > +               set_memory_rw(addr, area->nr_pages);
> 
> Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> want that alias gone before any deallocation happens".
> VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> me, please".  deallocate means "this was vfree -- please free the
> pages".  I'm not convinced that all the various combinations make
> sense.  Do we really need both flags?
VM_HAS_SPECIAL_PERMS is supposed to mean, like you said, "reset the direct map".
Where VM_IMMEDIATE_UNMAP means, the vmalloc allocation has extra capabilties
where we don't want to leave an enhanced capability TLB entry to the freed page.

I was trying to pick names that could apply more generally for potential future 
special memory capabilities. Today VM_HAS_SPECIAL_PERMS does just mean reset
write to the directmap and VM_IMMEDIATE_UNMAP means vmalloc mapping is
executable.

A present day reason for keeping both flags is, it is more efficient in the
arch-agnostic implementation when freeing memory that is just RO and not
executable. It saves a TLB flush.

> (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> not immediate.)
True, maybe VM_MUST_FLUSH or something else?

> If we do keep both flags, maybe some restructuring would make sense,
> like this, perhaps.  Sorry about horrible whitespace damage.
> 
> if (special) {
>   /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
>   WARN_ON_ONCE(!deallocate_pages);
> 
>   if (immediate) {
>     /* It's possible that the vmap alias is X and we're about to make
> the direct map RW.  To avoid a window where executable memory is
> writable, first mark the vmap alias NX.  This is silly, since we're
> about to *unmap* it, but this is the best we can do if all we have to
> work with is the set_memory_abc() APIs.  Architectures should override
> this whole function to get better behavior. */
>     set_memory_nx(...);
>   }
> 
>   set_memory_rw(addr, area->nr_pages);
> }
Ok.

> 
> > +
> > +       /* Always actually remove the area */
> > +       remove_vm_area(area->addr);
> > +
> > +       /*
> > +        * Need to flush the TLB before freeing pages in the case of this
> > flag.
> > +        * As long as that's happening, unmap aliases.
> > +        *
> > +        * For 2 and 3, this will not be needed because of the set_memory_nx
> > +        * above, because the stale TLBs will be NX.
> 
> I'm not sure I agree with this comment.  If the caller asked for an
> immediate unmap, we should give an immediate unmap.  But I'm still not
> sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
Yea. I was just trying to save a TLB flush, since for today's callers that have
set_memory there isn't a security downside I know of to just leaving it NX.
Maybe its not worth the tradeoff of confusion? Or I can clarify that in the
comment.

> > +        */
> > +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > +               vm_unmap_aliases();
> > +}
> > +
> >  static void __vunmap(const void *addr, int deallocate_pages)
> >  {
> >         struct vm_struct *area;
> > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int
> > deallocate_pages)
> >         debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> >         debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
> > 
> > -       remove_vm_area(addr);
> > +       arch_vunmap(area, deallocate_pages);
> > +
> >         if (deallocate_pages) {
> >                 int i;
> > 
> > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const void *addr)
> >          * nother cpu's list.  schedule_work() should be fine with this too.
> >          */
> >         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work),
> > GFP_ATOMIC);
> > +
> > +       /* If no memory for the deferred list node, give up */
> > +       if (!w)
> > +               return;
> 
> That's nasty.  I see what you're trying to do here, but I think you're
> solving a problem that doesn't need solving quite so urgently.  How
> about dropping this part and replacing it with a comment like "NB:
> this writes a word to a potentially executable address.  It would be
> nice if we could avoid doing this."  And maybe a future patch could
> more robustly avoid it without risking memory leaks.
Yea, sorry I should have called this out, because I wasn't sure on how likely
that was to happen. I did find some other places in the kernel with the same
ignoring logic.

I'll have to think though, I am not sure what the alternative is. Since the
memory can be RO in the module_memfree case, the old method of re-using the
allocation will no longer work. The list node could be stuffed on the vm_struct,
but then the all of the spin_lock(&vmap_area_lock)'s need to be changed to work
with interrupts so that the struct could be looked up. Not sure of the
implications of that. Or maybe have some slow backup that resets the permissions
and re-uses the allocation if kmalloc fails?

I guess it could also go back to the old v1 implementation that doesn't handle
RO and the directmap, and leave the W^X violation window during teardown. Then
solve that problem when modules are loaded via something like Nadav's stuff.

Andy Lutomirski Dec. 12, 2018, 7:57 p.m. UTC | #3

On Wed, Dec 12, 2018 at 11:50 AM Edgecombe, Rick P
<[e-mail]> wrote:
>
> On Tue, 2018-12-11 at 18:20 -0800, Andy Lutomirski wrote:
> > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > <[e-mail]> wrote:
> > >
> > > This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
> > > enabling vfree operations to immediately clear executable TLB entries to
> > > freed
> > > pages, and handle freeing memory with special permissions.
> > >
> > > In order to support vfree being called on memory that might be RO, the vfree
> > > deferred list node is moved to a kmalloc allocated struct, from where it is
> > > today, reusing the allocation being freed.
> > >
> > > arch_vunmap is a new __weak function that implements the actual unmapping
> > > and
> > > resetting of the direct map permissions. It can be overridden by more
> > > efficient
> > > architecture specific implementations.
> > >
> > > For the default implementation, it uses architecture agnostic methods which
> > > are
> > > equivalent to what most usages do before calling vfree. So now it is just
> > > centralized here.
> > >
> > > This implementation derives from two sketches from Dave Hansen and Andy
> > > Lutomirski.
> > >
> > > Suggested-by: Dave Hansen <[e-mail]>
> > > Suggested-by: Andy Lutomirski <[e-mail]>
> > > Suggested-by: Will Deacon <[e-mail]>
> > > Signed-off-by: Rick Edgecombe <[e-mail]>
> > > ---
> > >  include/linux/vmalloc.h |  2 ++
> > >  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
> > >  2 files changed, 69 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > > index 398e9c95cd61..872bcde17aca 100644
> > > --- a/include/linux/vmalloc.h
> > > +++ b/include/linux/vmalloc.h
> > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h */
> > >  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not fully
> > > initialized */
> > >  #define VM_NO_GUARD            0x00000040      /* don't add guard page */
> > >  #define VM_KASAN               0x00000080      /* has allocated kasan
> > > shadow memory */
> > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before releasing
> > > pages */
> > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with special
> > > perms */
> > >  /* bits [20..32] reserved for arch specific ioremap internals */
> > >
> > >  /*
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 97d4b25d0373..02b284d2245a 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -18,6 +18,7 @@
> > >  #include <linux/interrupt.h>
> > >  #include <linux/proc_fs.h>
> > >  #include <linux/seq_file.h>
> > > +#include <linux/set_memory.h>
> > >  #include <linux/debugobjects.h>
> > >  #include <linux/kallsyms.h>
> > >  #include <linux/list.h>
> > > @@ -38,6 +39,11 @@
> > >
> > >  #include "internal.h"
> > >
> > > +struct vfree_work {
> > > +       struct llist_node node;
> > > +       void *addr;
> > > +};
> > > +
> > >  struct vfree_deferred {
> > >         struct llist_head list;
> > >         struct work_struct wq;
> > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> > >  {
> > >         struct vfree_deferred *p = container_of(w, struct vfree_deferred,
> > > wq);
> > >         struct llist_node *t, *llnode;
> > > +       struct vfree_work *cur;
> > >
> > > -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> > > -               __vunmap((void *)llnode, 1);
> > > +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> > > +               cur = container_of(llnode, struct vfree_work, node);
> > > +               __vunmap(cur->addr, 1);
> > > +               kfree(cur);
> > > +       }
> > >  }
> > >
> > >  /*** Page table manipulation functions ***/
> > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void *addr)
> > >         return NULL;
> > >  }
> > >
> > > +/*
> > > + * This function handles unmapping and resetting the direct map as
> > > efficiently
> > > + * as it can with cross arch functions. The three categories of
> > > architectures
> > > + * are:
> > > + *   1. Architectures with no set_memory implementations and no direct map
> > > + *      permissions.
> > > + *   2. Architectures with set_memory implementations but no direct map
> > > + *      permissions
> > > + *   3. Architectures with set_memory implementations and direct map
> > > permissions
> > > + */
> > > +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
> >
> > My general preference is to avoid __weak functions -- they don't
> > optimize well.  Instead, I prefer either:
> >
> > #ifndef arch_vunmap
> > void arch_vunmap(...);
> > #endif
> >
> > or
> >
> > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > ...
> > #endif
> Ok.
> >
> > > +{
> > > +       unsigned long addr = (unsigned long)area->addr;
> > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > +
> > > +       /*
> > > +        * In case of 2 and 3, use this general way of resetting the
> > > permissions
> > > +        * on the directmap. Do NX before RW, in case of X, so there is no
> > > W^X
> > > +        * violation window.
> > > +        *
> > > +        * For case 1 these will be noops.
> > > +        */
> > > +       if (immediate)
> > > +               set_memory_nx(addr, area->nr_pages);
> > > +       if (deallocate_pages && special)
> > > +               set_memory_rw(addr, area->nr_pages);
> >
> > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> > want that alias gone before any deallocation happens".
> > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> > me, please".  deallocate means "this was vfree -- please free the
> > pages".  I'm not convinced that all the various combinations make
> > sense.  Do we really need both flags?
> VM_HAS_SPECIAL_PERMS is supposed to mean, like you said, "reset the direct map".
> Where VM_IMMEDIATE_UNMAP means, the vmalloc allocation has extra capabilties
> where we don't want to leave an enhanced capability TLB entry to the freed page.
>
> I was trying to pick names that could apply more generally for potential future
> special memory capabilities. Today VM_HAS_SPECIAL_PERMS does just mean reset
> write to the directmap and VM_IMMEDIATE_UNMAP means vmalloc mapping is
> executable.
>
> A present day reason for keeping both flags is, it is more efficient in the
> arch-agnostic implementation when freeing memory that is just RO and not
> executable. It saves a TLB flush.
>
> > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> > not immediate.)
> True, maybe VM_MUST_FLUSH or something else?
>
> > If we do keep both flags, maybe some restructuring would make sense,
> > like this, perhaps.  Sorry about horrible whitespace damage.
> >
> > if (special) {
> >   /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
> >   WARN_ON_ONCE(!deallocate_pages);
> >
> >   if (immediate) {
> >     /* It's possible that the vmap alias is X and we're about to make
> > the direct map RW.  To avoid a window where executable memory is
> > writable, first mark the vmap alias NX.  This is silly, since we're
> > about to *unmap* it, but this is the best we can do if all we have to
> > work with is the set_memory_abc() APIs.  Architectures should override
> > this whole function to get better behavior. */
> >     set_memory_nx(...);
> >   }
> >
> >   set_memory_rw(addr, area->nr_pages);
> > }
> Ok.
>
> >
> > > +
> > > +       /* Always actually remove the area */
> > > +       remove_vm_area(area->addr);
> > > +
> > > +       /*
> > > +        * Need to flush the TLB before freeing pages in the case of this
> > > flag.
> > > +        * As long as that's happening, unmap aliases.
> > > +        *
> > > +        * For 2 and 3, this will not be needed because of the set_memory_nx
> > > +        * above, because the stale TLBs will be NX.
> >
> > I'm not sure I agree with this comment.  If the caller asked for an
> > immediate unmap, we should give an immediate unmap.  But I'm still not
> > sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
> Yea. I was just trying to save a TLB flush, since for today's callers that have
> set_memory there isn't a security downside I know of to just leaving it NX.
> Maybe its not worth the tradeoff of confusion? Or I can clarify that in the
> comment.

Don't both of the users in your series set both flags, though?  My
real objection to having them be separate is that, in the absence of
users, it's less clear exactly what they should do and the code
doesn't get exercised.

If you document that VM_IMMEDIATE_UNMAP means "I want the TLB entries
gone", then I can re-review the code in light of that.  But then I'm
unconvinced by your generic implementation, since set_memory_nx()
seems like an odd way to go about it.

>
> > > +        */
> > > +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > > +               vm_unmap_aliases();
> > > +}
> > > +
> > >  static void __vunmap(const void *addr, int deallocate_pages)
> > >  {
> > >         struct vm_struct *area;
> > > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int
> > > deallocate_pages)
> > >         debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> > >         debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
> > >
> > > -       remove_vm_area(addr);
> > > +       arch_vunmap(area, deallocate_pages);
> > > +
> > >         if (deallocate_pages) {
> > >                 int i;
> > >
> > > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const void *addr)
> > >          * nother cpu's list.  schedule_work() should be fine with this too.
> > >          */
> > >         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > > +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work),
> > > GFP_ATOMIC);
> > > +
> > > +       /* If no memory for the deferred list node, give up */
> > > +       if (!w)
> > > +               return;
> >
> > That's nasty.  I see what you're trying to do here, but I think you're
> > solving a problem that doesn't need solving quite so urgently.  How
> > about dropping this part and replacing it with a comment like "NB:
> > this writes a word to a potentially executable address.  It would be
> > nice if we could avoid doing this."  And maybe a future patch could
> > more robustly avoid it without risking memory leaks.
> Yea, sorry I should have called this out, because I wasn't sure on how likely
> that was to happen. I did find some other places in the kernel with the same
> ignoring logic.
>
> I'll have to think though, I am not sure what the alternative is. Since the
> memory can be RO in the module_memfree case, the old method of re-using the
> allocation will no longer work. The list node could be stuffed on the vm_struct,
> but then the all of the spin_lock(&vmap_area_lock)'s need to be changed to work
> with interrupts so that the struct could be looked up. Not sure of the
> implications of that. Or maybe have some slow backup that resets the permissions
> and re-uses the allocation if kmalloc fails?
>
> I guess it could also go back to the old v1 implementation that doesn't handle
> RO and the directmap, and leave the W^X violation window during teardown. Then
> solve that problem when modules are loaded via something like Nadav's stuff.
>

Hmm.  Switching to spin_lock_irqsave() doesn't seem so bad to me.

Rick Edgecombe Dec. 12, 2018, 10:01 p.m. UTC | #4

On Wed, 2018-12-12 at 11:57 -0800, Andy Lutomirski wrote:
> On Wed, Dec 12, 2018 at 11:50 AM Edgecombe, Rick P
> <[e-mail]> wrote:
> > 
> > On Tue, 2018-12-11 at 18:20 -0800, Andy Lutomirski wrote:
> > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > <[e-mail]> wrote:
> > > > 
> > > > This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
> > > > enabling vfree operations to immediately clear executable TLB entries to
> > > > freed
> > > > pages, and handle freeing memory with special permissions.
> > > > 
> > > > In order to support vfree being called on memory that might be RO, the
> > > > vfree
> > > > deferred list node is moved to a kmalloc allocated struct, from where it
> > > > is
> > > > today, reusing the allocation being freed.
> > > > 
> > > > arch_vunmap is a new __weak function that implements the actual
> > > > unmapping
> > > > and
> > > > resetting of the direct map permissions. It can be overridden by more
> > > > efficient
> > > > architecture specific implementations.
> > > > 
> > > > For the default implementation, it uses architecture agnostic methods
> > > > which
> > > > are
> > > > equivalent to what most usages do before calling vfree. So now it is
> > > > just
> > > > centralized here.
> > > > 
> > > > This implementation derives from two sketches from Dave Hansen and Andy
> > > > Lutomirski.
> > > > 
> > > > Suggested-by: Dave Hansen <[e-mail]>
> > > > Suggested-by: Andy Lutomirski <[e-mail]>
> > > > Suggested-by: Will Deacon <[e-mail]>
> > > > Signed-off-by: Rick Edgecombe <[e-mail]>
> > > > ---
> > > >  include/linux/vmalloc.h |  2 ++
> > > >  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
> > > >  2 files changed, 69 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > > > index 398e9c95cd61..872bcde17aca 100644
> > > > --- a/include/linux/vmalloc.h
> > > > +++ b/include/linux/vmalloc.h
> > > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h
> > > > */
> > > >  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not
> > > > fully
> > > > initialized */
> > > >  #define VM_NO_GUARD            0x00000040      /* don't add guard page
> > > > */
> > > >  #define VM_KASAN               0x00000080      /* has allocated kasan
> > > > shadow memory */
> > > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before
> > > > releasing
> > > > pages */
> > > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with
> > > > special
> > > > perms */
> > > >  /* bits [20..32] reserved for arch specific ioremap internals */
> > > > 
> > > >  /*
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index 97d4b25d0373..02b284d2245a 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -18,6 +18,7 @@
> > > >  #include <linux/interrupt.h>
> > > >  #include <linux/proc_fs.h>
> > > >  #include <linux/seq_file.h>
> > > > +#include <linux/set_memory.h>
> > > >  #include <linux/debugobjects.h>
> > > >  #include <linux/kallsyms.h>
> > > >  #include <linux/list.h>
> > > > @@ -38,6 +39,11 @@
> > > > 
> > > >  #include "internal.h"
> > > > 
> > > > +struct vfree_work {
> > > > +       struct llist_node node;
> > > > +       void *addr;
> > > > +};
> > > > +
> > > >  struct vfree_deferred {
> > > >         struct llist_head list;
> > > >         struct work_struct wq;
> > > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> > > >  {
> > > >         struct vfree_deferred *p = container_of(w, struct
> > > > vfree_deferred,
> > > > wq);
> > > >         struct llist_node *t, *llnode;
> > > > +       struct vfree_work *cur;
> > > > 
> > > > -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> > > > -               __vunmap((void *)llnode, 1);
> > > > +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> > > > +               cur = container_of(llnode, struct vfree_work, node);
> > > > +               __vunmap(cur->addr, 1);
> > > > +               kfree(cur);
> > > > +       }
> > > >  }
> > > > 
> > > >  /*** Page table manipulation functions ***/
> > > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void
> > > > *addr)
> > > >         return NULL;
> > > >  }
> > > > 
> > > > +/*
> > > > + * This function handles unmapping and resetting the direct map as
> > > > efficiently
> > > > + * as it can with cross arch functions. The three categories of
> > > > architectures
> > > > + * are:
> > > > + *   1. Architectures with no set_memory implementations and no direct
> > > > map
> > > > + *      permissions.
> > > > + *   2. Architectures with set_memory implementations but no direct map
> > > > + *      permissions
> > > > + *   3. Architectures with set_memory implementations and direct map
> > > > permissions
> > > > + */
> > > > +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
> > > 
> > > My general preference is to avoid __weak functions -- they don't
> > > optimize well.  Instead, I prefer either:
> > > 
> > > #ifndef arch_vunmap
> > > void arch_vunmap(...);
> > > #endif
> > > 
> > > or
> > > 
> > > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > > ...
> > > #endif
> > 
> > Ok.
> > > 
> > > > +{
> > > > +       unsigned long addr = (unsigned long)area->addr;
> > > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > > +
> > > > +       /*
> > > > +        * In case of 2 and 3, use this general way of resetting the
> > > > permissions
> > > > +        * on the directmap. Do NX before RW, in case of X, so there is
> > > > no
> > > > W^X
> > > > +        * violation window.
> > > > +        *
> > > > +        * For case 1 these will be noops.
> > > > +        */
> > > > +       if (immediate)
> > > > +               set_memory_nx(addr, area->nr_pages);
> > > > +       if (deallocate_pages && special)
> > > > +               set_memory_rw(addr, area->nr_pages);
> > > 
> > > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> > > want that alias gone before any deallocation happens".
> > > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> > > me, please".  deallocate means "this was vfree -- please free the
> > > pages".  I'm not convinced that all the various combinations make
> > > sense.  Do we really need both flags?
> > 
> > VM_HAS_SPECIAL_PERMS is supposed to mean, like you said, "reset the direct
> > map".
> > Where VM_IMMEDIATE_UNMAP means, the vmalloc allocation has extra capabilties
> > where we don't want to leave an enhanced capability TLB entry to the freed
> > page.
> > 
> > I was trying to pick names that could apply more generally for potential
> > future
> > special memory capabilities. Today VM_HAS_SPECIAL_PERMS does just mean reset
> > write to the directmap and VM_IMMEDIATE_UNMAP means vmalloc mapping is
> > executable.
> > 
> > A present day reason for keeping both flags is, it is more efficient in the
> > arch-agnostic implementation when freeing memory that is just RO and not
> > executable. It saves a TLB flush.
> > 
> > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> > > not immediate.)
> > 
> > True, maybe VM_MUST_FLUSH or something else?
> > 
> > > If we do keep both flags, maybe some restructuring would make sense,
> > > like this, perhaps.  Sorry about horrible whitespace damage.
> > > 
> > > if (special) {
> > >   /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
> > >   WARN_ON_ONCE(!deallocate_pages);
> > > 
> > >   if (immediate) {
> > >     /* It's possible that the vmap alias is X and we're about to make
> > > the direct map RW.  To avoid a window where executable memory is
> > > writable, first mark the vmap alias NX.  This is silly, since we're
> > > about to *unmap* it, but this is the best we can do if all we have to
> > > work with is the set_memory_abc() APIs.  Architectures should override
> > > this whole function to get better behavior. */
> > >     set_memory_nx(...);
> > >   }
> > > 
> > >   set_memory_rw(addr, area->nr_pages);
> > > }
> > 
> > Ok.
> > 
> > > 
> > > > +
> > > > +       /* Always actually remove the area */
> > > > +       remove_vm_area(area->addr);
> > > > +
> > > > +       /*
> > > > +        * Need to flush the TLB before freeing pages in the case of
> > > > this
> > > > flag.
> > > > +        * As long as that's happening, unmap aliases.
> > > > +        *
> > > > +        * For 2 and 3, this will not be needed because of the
> > > > set_memory_nx
> > > > +        * above, because the stale TLBs will be NX.
> > > 
> > > I'm not sure I agree with this comment.  If the caller asked for an
> > > immediate unmap, we should give an immediate unmap.  But I'm still not
> > > sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
> > 
> > Yea. I was just trying to save a TLB flush, since for today's callers that
> > have
> > set_memory there isn't a security downside I know of to just leaving it NX.
> > Maybe its not worth the tradeoff of confusion? Or I can clarify that in the
> > comment.
> 
> Don't both of the users in your series set both flags, though?  My
> real objection to having them be separate is that, in the absence of
> users, it's less clear exactly what they should do and the code
> doesn't get exercised.
The only "just RO" user today is one of the BPF allocations. I don't have a
strong objection to combining them, just explaining the thinking. I guess if we
could always add another flag later if it becomes more needed.

> If you document that VM_IMMEDIATE_UNMAP means "I want the TLB entries
> gone", then I can re-review the code in light of that.  But then I'm
> unconvinced by your generic implementation, since set_memory_nx()
> seems like an odd way to go about it.
Masami Hiramatsu pointed out if we don't do set_memory_nx before set_memory_rw,
then there will be a small window of W^X violation. So that was the concern for
the executable case, regardless of the semantics. I think the concern applies
for any "special capability" permissions. Alternatively, if we remove_vm_area
before we reset the direct map perms RW, maybe that would accomplish the same
thing, if that's possible in a cross arch way. Maybe this is too much designing
for hypothetical future... just was trying to avoid having to change the
interface, and could just update the generic implementation if new permissions
or usages come up.

The set_memory_ stuff is really only needed for arm64 which seems to be the only
other one with directmap permissions. So if it could eventually have its own
arch_vunmap then all of the set_memory_ parts could be dropped and the default
would just be the simple unmap then flush logic that it was originally.

Or we have up to three flushes for the generic version and meet the name
expectations and needed functionality today. I guess I'll just try that.
> > 
> > > > +        */
> > > > +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > > > +               vm_unmap_aliases();
> > > > +}
> > > > +
> > > >  static void __vunmap(const void *addr, int deallocate_pages)
> > > >  {
> > > >         struct vm_struct *area;
> > > > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int
> > > > deallocate_pages)
> > > >         debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> > > >         debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
> > > > 
> > > > -       remove_vm_area(addr);
> > > > +       arch_vunmap(area, deallocate_pages);
> > > > +
> > > >         if (deallocate_pages) {
> > > >                 int i;
> > > > 
> > > > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const void
> > > > *addr)
> > > >          * nother cpu's list.  schedule_work() should be fine with this
> > > > too.
> > > >          */
> > > >         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > > > +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work),
> > > > GFP_ATOMIC);
> > > > +
> > > > +       /* If no memory for the deferred list node, give up */
> > > > +       if (!w)
> > > > +               return;
> > > 
> > > That's nasty.  I see what you're trying to do here, but I think you're
> > > solving a problem that doesn't need solving quite so urgently.  How
> > > about dropping this part and replacing it with a comment like "NB:
> > > this writes a word to a potentially executable address.  It would be
> > > nice if we could avoid doing this."  And maybe a future patch could
> > > more robustly avoid it without risking memory leaks.
> > 
> > Yea, sorry I should have called this out, because I wasn't sure on how
> > likely
> > that was to happen. I did find some other places in the kernel with the same
> > ignoring logic.
> > 
> > I'll have to think though, I am not sure what the alternative is. Since the
> > memory can be RO in the module_memfree case, the old method of re-using the
> > allocation will no longer work. The list node could be stuffed on the
> > vm_struct,
> > but then the all of the spin_lock(&vmap_area_lock)'s need to be changed to
> > work
> > with interrupts so that the struct could be looked up. Not sure of the
> > implications of that. Or maybe have some slow backup that resets the
> > permissions
> > and re-uses the allocation if kmalloc fails?
> > 
> > I guess it could also go back to the old v1 implementation that doesn't
> > handle
> > RO and the directmap, and leave the W^X violation window during teardown.
> > Then
> > solve that problem when modules are loaded via something like Nadav's stuff.
> > 
> 
> Hmm.  Switching to spin_lock_irqsave() doesn't seem so bad to me.
Ok.

Andy Lutomirski Dec. 15, 2018, 6:52 p.m. UTC | #5

On Wed, Dec 12, 2018 at 2:01 PM Edgecombe, Rick P
<[e-mail]> wrote:
>
> On Wed, 2018-12-12 at 11:57 -0800, Andy Lutomirski wrote:
> > On Wed, Dec 12, 2018 at 11:50 AM Edgecombe, Rick P
> > <[e-mail]> wrote:
> > >
> > > On Tue, 2018-12-11 at 18:20 -0800, Andy Lutomirski wrote:
> > > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > > <[e-mail]> wrote:
> > > > >
> > > > > This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
> > > > > enabling vfree operations to immediately clear executable TLB entries to
> > > > > freed
> > > > > pages, and handle freeing memory with special permissions.
> > > > >
> > > > > In order to support vfree being called on memory that might be RO, the
> > > > > vfree
> > > > > deferred list node is moved to a kmalloc allocated struct, from where it
> > > > > is
> > > > > today, reusing the allocation being freed.
> > > > >
> > > > > arch_vunmap is a new __weak function that implements the actual
> > > > > unmapping
> > > > > and
> > > > > resetting of the direct map permissions. It can be overridden by more
> > > > > efficient
> > > > > architecture specific implementations.
> > > > >
> > > > > For the default implementation, it uses architecture agnostic methods
> > > > > which
> > > > > are
> > > > > equivalent to what most usages do before calling vfree. So now it is
> > > > > just
> > > > > centralized here.
> > > > >
> > > > > This implementation derives from two sketches from Dave Hansen and Andy
> > > > > Lutomirski.
> > > > >
> > > > > Suggested-by: Dave Hansen <[e-mail]>
> > > > > Suggested-by: Andy Lutomirski <[e-mail]>
> > > > > Suggested-by: Will Deacon <[e-mail]>
> > > > > Signed-off-by: Rick Edgecombe <[e-mail]>
> > > > > ---
> > > > >  include/linux/vmalloc.h |  2 ++
> > > > >  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
> > > > >  2 files changed, 69 insertions(+), 6 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > > > > index 398e9c95cd61..872bcde17aca 100644
> > > > > --- a/include/linux/vmalloc.h
> > > > > +++ b/include/linux/vmalloc.h
> > > > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h
> > > > > */
> > > > >  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not
> > > > > fully
> > > > > initialized */
> > > > >  #define VM_NO_GUARD            0x00000040      /* don't add guard page
> > > > > */
> > > > >  #define VM_KASAN               0x00000080      /* has allocated kasan
> > > > > shadow memory */
> > > > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before
> > > > > releasing
> > > > > pages */
> > > > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with
> > > > > special
> > > > > perms */
> > > > >  /* bits [20..32] reserved for arch specific ioremap internals */
> > > > >
> > > > >  /*
> > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > index 97d4b25d0373..02b284d2245a 100644
> > > > > --- a/mm/vmalloc.c
> > > > > +++ b/mm/vmalloc.c
> > > > > @@ -18,6 +18,7 @@
> > > > >  #include <linux/interrupt.h>
> > > > >  #include <linux/proc_fs.h>
> > > > >  #include <linux/seq_file.h>
> > > > > +#include <linux/set_memory.h>
> > > > >  #include <linux/debugobjects.h>
> > > > >  #include <linux/kallsyms.h>
> > > > >  #include <linux/list.h>
> > > > > @@ -38,6 +39,11 @@
> > > > >
> > > > >  #include "internal.h"
> > > > >
> > > > > +struct vfree_work {
> > > > > +       struct llist_node node;
> > > > > +       void *addr;
> > > > > +};
> > > > > +
> > > > >  struct vfree_deferred {
> > > > >         struct llist_head list;
> > > > >         struct work_struct wq;
> > > > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> > > > >  {
> > > > >         struct vfree_deferred *p = container_of(w, struct
> > > > > vfree_deferred,
> > > > > wq);
> > > > >         struct llist_node *t, *llnode;
> > > > > +       struct vfree_work *cur;
> > > > >
> > > > > -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> > > > > -               __vunmap((void *)llnode, 1);
> > > > > +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> > > > > +               cur = container_of(llnode, struct vfree_work, node);
> > > > > +               __vunmap(cur->addr, 1);
> > > > > +               kfree(cur);
> > > > > +       }
> > > > >  }
> > > > >
> > > > >  /*** Page table manipulation functions ***/
> > > > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void
> > > > > *addr)
> > > > >         return NULL;
> > > > >  }
> > > > >
> > > > > +/*
> > > > > + * This function handles unmapping and resetting the direct map as
> > > > > efficiently
> > > > > + * as it can with cross arch functions. The three categories of
> > > > > architectures
> > > > > + * are:
> > > > > + *   1. Architectures with no set_memory implementations and no direct
> > > > > map
> > > > > + *      permissions.
> > > > > + *   2. Architectures with set_memory implementations but no direct map
> > > > > + *      permissions
> > > > > + *   3. Architectures with set_memory implementations and direct map
> > > > > permissions
> > > > > + */
> > > > > +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
> > > >
> > > > My general preference is to avoid __weak functions -- they don't
> > > > optimize well.  Instead, I prefer either:
> > > >
> > > > #ifndef arch_vunmap
> > > > void arch_vunmap(...);
> > > > #endif
> > > >
> > > > or
> > > >
> > > > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > > > ...
> > > > #endif
> > >
> > > Ok.
> > > >
> > > > > +{
> > > > > +       unsigned long addr = (unsigned long)area->addr;
> > > > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > > > +
> > > > > +       /*
> > > > > +        * In case of 2 and 3, use this general way of resetting the
> > > > > permissions
> > > > > +        * on the directmap. Do NX before RW, in case of X, so there is
> > > > > no
> > > > > W^X
> > > > > +        * violation window.
> > > > > +        *
> > > > > +        * For case 1 these will be noops.
> > > > > +        */
> > > > > +       if (immediate)
> > > > > +               set_memory_nx(addr, area->nr_pages);
> > > > > +       if (deallocate_pages && special)
> > > > > +               set_memory_rw(addr, area->nr_pages);
> > > >
> > > > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> > > > want that alias gone before any deallocation happens".
> > > > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> > > > me, please".  deallocate means "this was vfree -- please free the
> > > > pages".  I'm not convinced that all the various combinations make
> > > > sense.  Do we really need both flags?
> > >
> > > VM_HAS_SPECIAL_PERMS is supposed to mean, like you said, "reset the direct
> > > map".
> > > Where VM_IMMEDIATE_UNMAP means, the vmalloc allocation has extra capabilties
> > > where we don't want to leave an enhanced capability TLB entry to the freed
> > > page.
> > >
> > > I was trying to pick names that could apply more generally for potential
> > > future
> > > special memory capabilities. Today VM_HAS_SPECIAL_PERMS does just mean reset
> > > write to the directmap and VM_IMMEDIATE_UNMAP means vmalloc mapping is
> > > executable.
> > >
> > > A present day reason for keeping both flags is, it is more efficient in the
> > > arch-agnostic implementation when freeing memory that is just RO and not
> > > executable. It saves a TLB flush.
> > >
> > > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> > > > not immediate.)
> > >
> > > True, maybe VM_MUST_FLUSH or something else?
> > >
> > > > If we do keep both flags, maybe some restructuring would make sense,
> > > > like this, perhaps.  Sorry about horrible whitespace damage.
> > > >
> > > > if (special) {
> > > >   /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
> > > >   WARN_ON_ONCE(!deallocate_pages);
> > > >
> > > >   if (immediate) {
> > > >     /* It's possible that the vmap alias is X and we're about to make
> > > > the direct map RW.  To avoid a window where executable memory is
> > > > writable, first mark the vmap alias NX.  This is silly, since we're
> > > > about to *unmap* it, but this is the best we can do if all we have to
> > > > work with is the set_memory_abc() APIs.  Architectures should override
> > > > this whole function to get better behavior. */
> > > >     set_memory_nx(...);
> > > >   }
> > > >
> > > >   set_memory_rw(addr, area->nr_pages);
> > > > }
> > >
> > > Ok.
> > >
> > > >
> > > > > +
> > > > > +       /* Always actually remove the area */
> > > > > +       remove_vm_area(area->addr);
> > > > > +
> > > > > +       /*
> > > > > +        * Need to flush the TLB before freeing pages in the case of
> > > > > this
> > > > > flag.
> > > > > +        * As long as that's happening, unmap aliases.
> > > > > +        *
> > > > > +        * For 2 and 3, this will not be needed because of the
> > > > > set_memory_nx
> > > > > +        * above, because the stale TLBs will be NX.
> > > >
> > > > I'm not sure I agree with this comment.  If the caller asked for an
> > > > immediate unmap, we should give an immediate unmap.  But I'm still not
> > > > sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
> > >
> > > Yea. I was just trying to save a TLB flush, since for today's callers that
> > > have
> > > set_memory there isn't a security downside I know of to just leaving it NX.
> > > Maybe its not worth the tradeoff of confusion? Or I can clarify that in the
> > > comment.
> >
> > Don't both of the users in your series set both flags, though?  My
> > real objection to having them be separate is that, in the absence of
> > users, it's less clear exactly what they should do and the code
> > doesn't get exercised.
> The only "just RO" user today is one of the BPF allocations. I don't have a
> strong objection to combining them, just explaining the thinking. I guess if we
> could always add another flag later if it becomes more needed.
>
> > If you document that VM_IMMEDIATE_UNMAP means "I want the TLB entries
> > gone", then I can re-review the code in light of that.  But then I'm
> > unconvinced by your generic implementation, since set_memory_nx()
> > seems like an odd way to go about it.
> Masami Hiramatsu pointed out if we don't do set_memory_nx before set_memory_rw,
> then there will be a small window of W^X violation. So that was the concern for
> the executable case, regardless of the semantics. I think the concern applies
> for any "special capability" permissions. Alternatively, if we remove_vm_area
> before we reset the direct map perms RW, maybe that would accomplish the same
> thing, if that's possible in a cross arch way. Maybe this is too much designing
> for hypothetical future... just was trying to avoid having to change the
> interface, and could just update the generic implementation if new permissions
> or usages come up.
>
> The set_memory_ stuff is really only needed for arm64 which seems to be the only
> other one with directmap permissions. So if it could eventually have its own
> arch_vunmap then all of the set_memory_ parts could be dropped and the default
> would just be the simple unmap then flush logic that it was originally.

I think that's probably the best solution.  If there are only two
arches that have anything fancy here, let's just fix both of them up
for real.

>
> Or we have up to three flushes for the generic version and meet the name
> expectations and needed functionality today. I guess I'll just try that.
> > >
> > > > > +        */
> > > > > +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > > > > +               vm_unmap_aliases();
> > > > > +}
> > > > > +
> > > > >  static void __vunmap(const void *addr, int deallocate_pages)
> > > > >  {
> > > > >         struct vm_struct *area;
> > > > > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int
> > > > > deallocate_pages)
> > > > >         debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> > > > >         debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
> > > > >
> > > > > -       remove_vm_area(addr);
> > > > > +       arch_vunmap(area, deallocate_pages);
> > > > > +
> > > > >         if (deallocate_pages) {
> > > > >                 int i;
> > > > >
> > > > > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const void
> > > > > *addr)
> > > > >          * nother cpu's list.  schedule_work() should be fine with this
> > > > > too.
> > > > >          */
> > > > >         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > > > > +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work),
> > > > > GFP_ATOMIC);
> > > > > +
> > > > > +       /* If no memory for the deferred list node, give up */
> > > > > +       if (!w)
> > > > > +               return;
> > > >
> > > > That's nasty.  I see what you're trying to do here, but I think you're
> > > > solving a problem that doesn't need solving quite so urgently.  How
> > > > about dropping this part and replacing it with a comment like "NB:
> > > > this writes a word to a potentially executable address.  It would be
> > > > nice if we could avoid doing this."  And maybe a future patch could
> > > > more robustly avoid it without risking memory leaks.
> > >
> > > Yea, sorry I should have called this out, because I wasn't sure on how
> > > likely
> > > that was to happen. I did find some other places in the kernel with the same
> > > ignoring logic.
> > >
> > > I'll have to think though, I am not sure what the alternative is. Since the
> > > memory can be RO in the module_memfree case, the old method of re-using the
> > > allocation will no longer work. The list node could be stuffed on the
> > > vm_struct,
> > > but then the all of the spin_lock(&vmap_area_lock)'s need to be changed to
> > > work
> > > with interrupts so that the struct could be looked up. Not sure of the
> > > implications of that. Or maybe have some slow backup that resets the
> > > permissions
> > > and re-uses the allocation if kmalloc fails?
> > >
> > > I guess it could also go back to the old v1 implementation that doesn't
> > > handle
> > > RO and the directmap, and leave the W^X violation window during teardown.
> > > Then
> > > solve that problem when modules are loaded via something like Nadav's stuff.
> > >
> >
> > Hmm.  Switching to spin_lock_irqsave() doesn't seem so bad to me.
> Ok.

Actually, I think I have a better solution.  Just declare the
problematic case to be illegal: say that you may not free memory with
the new flags set while IRQs are off.  Enforce this with a VM_WARN_ON
in the code that reads the vfree_deferred list.

Rick Edgecombe Dec. 18, 2018, 12:23 a.m. UTC | #6

On Sat, 2018-12-15 at 10:52 -0800, Andy Lutomirski wrote:
> On Wed, Dec 12, 2018 at 2:01 PM Edgecombe, Rick P
> <[e-mail]> wrote:
> > 
> > On Wed, 2018-12-12 at 11:57 -0800, Andy Lutomirski wrote:
> > > On Wed, Dec 12, 2018 at 11:50 AM Edgecombe, Rick P
> > > <[e-mail]> wrote:
> > > > 
> > > > On Tue, 2018-12-11 at 18:20 -0800, Andy Lutomirski wrote:
> > > > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > > > <[e-mail]> wrote:
> > > > > > 
> > > > > > This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS,
> > > > > > for
> > > > > > enabling vfree operations to immediately clear executable TLB
> > > > > > entries to
> > > > > > freed
> > > > > > pages, and handle freeing memory with special permissions.
> > > > > > 
> > > > > > In order to support vfree being called on memory that might be RO,
> > > > > > the
> > > > > > vfree
> > > > > > deferred list node is moved to a kmalloc allocated struct, from
> > > > > > where it
> > > > > > is
> > > > > > today, reusing the allocation being freed.
> > > > > > 
> > > > > > arch_vunmap is a new __weak function that implements the actual
> > > > > > unmapping
> > > > > > and
> > > > > > resetting of the direct map permissions. It can be overridden by
> > > > > > more
> > > > > > efficient
> > > > > > architecture specific implementations.
> > > > > > 
> > > > > > For the default implementation, it uses architecture agnostic
> > > > > > methods
> > > > > > which
> > > > > > are
> > > > > > equivalent to what most usages do before calling vfree. So now it is
> > > > > > just
> > > > > > centralized here.
> > > > > > 
> > > > > > This implementation derives from two sketches from Dave Hansen and
> > > > > > Andy
> > > > > > Lutomirski.
> > > > > > 
> > > > > > Suggested-by: Dave Hansen <[e-mail]>
> > > > > > Suggested-by: Andy Lutomirski <[e-mail]>
> > > > > > Suggested-by: Will Deacon <[e-mail]>
> > > > > > Signed-off-by: Rick Edgecombe <[e-mail]>
> > > > > > ---
> > > > > >  include/linux/vmalloc.h |  2 ++
> > > > > >  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++-
> > > > > > ---
> > > > > >  2 files changed, 69 insertions(+), 6 deletions(-)
> > > > > > 
> > > > > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > > > > > index 398e9c95cd61..872bcde17aca 100644
> > > > > > --- a/include/linux/vmalloc.h
> > > > > > +++ b/include/linux/vmalloc.h
> > > > > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in
> > > > > > notifier.h
> > > > > > */
> > > > > >  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not
> > > > > > fully
> > > > > > initialized */
> > > > > >  #define VM_NO_GUARD            0x00000040      /* don't add guard
> > > > > > page
> > > > > > */
> > > > > >  #define VM_KASAN               0x00000080      /* has allocated
> > > > > > kasan
> > > > > > shadow memory */
> > > > > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before
> > > > > > releasing
> > > > > > pages */
> > > > > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with
> > > > > > special
> > > > > > perms */
> > > > > >  /* bits [20..32] reserved for arch specific ioremap internals */
> > > > > > 
> > > > > >  /*
> > > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > > index 97d4b25d0373..02b284d2245a 100644
> > > > > > --- a/mm/vmalloc.c
> > > > > > +++ b/mm/vmalloc.c
> > > > > > @@ -18,6 +18,7 @@
> > > > > >  #include <linux/interrupt.h>
> > > > > >  #include <linux/proc_fs.h>
> > > > > >  #include <linux/seq_file.h>
> > > > > > +#include <linux/set_memory.h>
> > > > > >  #include <linux/debugobjects.h>
> > > > > >  #include <linux/kallsyms.h>
> > > > > >  #include <linux/list.h>
> > > > > > @@ -38,6 +39,11 @@
> > > > > > 
> > > > > >  #include "internal.h"
> > > > > > 
> > > > > > +struct vfree_work {
> > > > > > +       struct llist_node node;
> > > > > > +       void *addr;
> > > > > > +};
> > > > > > +
> > > > > >  struct vfree_deferred {
> > > > > >         struct llist_head list;
> > > > > >         struct work_struct wq;
> > > > > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> > > > > >  {
> > > > > >         struct vfree_deferred *p = container_of(w, struct
> > > > > > vfree_deferred,
> > > > > > wq);
> > > > > >         struct llist_node *t, *llnode;
> > > > > > +       struct vfree_work *cur;
> > > > > > 
> > > > > > -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> > > > > > -               __vunmap((void *)llnode, 1);
> > > > > > +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> > > > > > +               cur = container_of(llnode, struct vfree_work, node);
> > > > > > +               __vunmap(cur->addr, 1);
> > > > > > +               kfree(cur);
> > > > > > +       }
> > > > > >  }
> > > > > > 
> > > > > >  /*** Page table manipulation functions ***/
> > > > > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void
> > > > > > *addr)
> > > > > >         return NULL;
> > > > > >  }
> > > > > > 
> > > > > > +/*
> > > > > > + * This function handles unmapping and resetting the direct map as
> > > > > > efficiently
> > > > > > + * as it can with cross arch functions. The three categories of
> > > > > > architectures
> > > > > > + * are:
> > > > > > + *   1. Architectures with no set_memory implementations and no
> > > > > > direct
> > > > > > map
> > > > > > + *      permissions.
> > > > > > + *   2. Architectures with set_memory implementations but no direct
> > > > > > map
> > > > > > + *      permissions
> > > > > > + *   3. Architectures with set_memory implementations and direct
> > > > > > map
> > > > > > permissions
> > > > > > + */
> > > > > > +void __weak arch_vunmap(struct vm_struct *area, int
> > > > > > deallocate_pages)
> > > > > 
> > > > > My general preference is to avoid __weak functions -- they don't
> > > > > optimize well.  Instead, I prefer either:
> > > > > 
> > > > > #ifndef arch_vunmap
> > > > > void arch_vunmap(...);
> > > > > #endif
> > > > > 
> > > > > or
> > > > > 
> > > > > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > > > > ...
> > > > > #endif
> > > > 
> > > > Ok.
> > > > > 
> > > > > > +{
> > > > > > +       unsigned long addr = (unsigned long)area->addr;
> > > > > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > > > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > > > > +
> > > > > > +       /*
> > > > > > +        * In case of 2 and 3, use this general way of resetting the
> > > > > > permissions
> > > > > > +        * on the directmap. Do NX before RW, in case of X, so there
> > > > > > is
> > > > > > no
> > > > > > W^X
> > > > > > +        * violation window.
> > > > > > +        *
> > > > > > +        * For case 1 these will be noops.
> > > > > > +        */
> > > > > > +       if (immediate)
> > > > > > +               set_memory_nx(addr, area->nr_pages);
> > > > > > +       if (deallocate_pages && special)
> > > > > > +               set_memory_rw(addr, area->nr_pages);
> > > > > 
> > > > > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> > > > > want that alias gone before any deallocation happens".
> > > > > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> > > > > me, please".  deallocate means "this was vfree -- please free the
> > > > > pages".  I'm not convinced that all the various combinations make
> > > > > sense.  Do we really need both flags?
> > > > 
> > > > VM_HAS_SPECIAL_PERMS is supposed to mean, like you said, "reset the
> > > > direct
> > > > map".
> > > > Where VM_IMMEDIATE_UNMAP means, the vmalloc allocation has extra
> > > > capabilties
> > > > where we don't want to leave an enhanced capability TLB entry to the
> > > > freed
> > > > page.
> > > > 
> > > > I was trying to pick names that could apply more generally for potential
> > > > future
> > > > special memory capabilities. Today VM_HAS_SPECIAL_PERMS does just mean
> > > > reset
> > > > write to the directmap and VM_IMMEDIATE_UNMAP means vmalloc mapping is
> > > > executable.
> > > > 
> > > > A present day reason for keeping both flags is, it is more efficient in
> > > > the
> > > > arch-agnostic implementation when freeing memory that is just RO and not
> > > > executable. It saves a TLB flush.
> > > > 
> > > > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> > > > > not immediate.)
> > > > 
> > > > True, maybe VM_MUST_FLUSH or something else?
> > > > 
> > > > > If we do keep both flags, maybe some restructuring would make sense,
> > > > > like this, perhaps.  Sorry about horrible whitespace damage.
> > > > > 
> > > > > if (special) {
> > > > >   /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages.
> > > > > */
> > > > >   WARN_ON_ONCE(!deallocate_pages);
> > > > > 
> > > > >   if (immediate) {
> > > > >     /* It's possible that the vmap alias is X and we're about to make
> > > > > the direct map RW.  To avoid a window where executable memory is
> > > > > writable, first mark the vmap alias NX.  This is silly, since we're
> > > > > about to *unmap* it, but this is the best we can do if all we have to
> > > > > work with is the set_memory_abc() APIs.  Architectures should override
> > > > > this whole function to get better behavior. */
> > > > >     set_memory_nx(...);
> > > > >   }
> > > > > 
> > > > >   set_memory_rw(addr, area->nr_pages);
> > > > > }
> > > > 
> > > > Ok.
> > > > 
> > > > > 
> > > > > > +
> > > > > > +       /* Always actually remove the area */
> > > > > > +       remove_vm_area(area->addr);
> > > > > > +
> > > > > > +       /*
> > > > > > +        * Need to flush the TLB before freeing pages in the case of
> > > > > > this
> > > > > > flag.
> > > > > > +        * As long as that's happening, unmap aliases.
> > > > > > +        *
> > > > > > +        * For 2 and 3, this will not be needed because of the
> > > > > > set_memory_nx
> > > > > > +        * above, because the stale TLBs will be NX.
> > > > > 
> > > > > I'm not sure I agree with this comment.  If the caller asked for an
> > > > > immediate unmap, we should give an immediate unmap.  But I'm still not
> > > > > sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
> > > > 
> > > > Yea. I was just trying to save a TLB flush, since for today's callers
> > > > that
> > > > have
> > > > set_memory there isn't a security downside I know of to just leaving it
> > > > NX.
> > > > Maybe its not worth the tradeoff of confusion? Or I can clarify that in
> > > > the
> > > > comment.
> > > 
> > > Don't both of the users in your series set both flags, though?  My
> > > real objection to having them be separate is that, in the absence of
> > > users, it's less clear exactly what they should do and the code
> > > doesn't get exercised.
> > 
> > The only "just RO" user today is one of the BPF allocations. I don't have a
> > strong objection to combining them, just explaining the thinking. I guess if
> > we
> > could always add another flag later if it becomes more needed.
> > 
> > > If you document that VM_IMMEDIATE_UNMAP means "I want the TLB entries
> > > gone", then I can re-review the code in light of that.  But then I'm
> > > unconvinced by your generic implementation, since set_memory_nx()
> > > seems like an odd way to go about it.
> > 
> > Masami Hiramatsu pointed out if we don't do set_memory_nx before
> > set_memory_rw,
> > then there will be a small window of W^X violation. So that was the concern
> > for
> > the executable case, regardless of the semantics. I think the concern
> > applies
> > for any "special capability" permissions. Alternatively, if we
> > remove_vm_area
> > before we reset the direct map perms RW, maybe that would accomplish the
> > same
> > thing, if that's possible in a cross arch way. Maybe this is too much
> > designing
> > for hypothetical future... just was trying to avoid having to change the
> > interface, and could just update the generic implementation if new
> > permissions
> > or usages come up.
> > 
> > The set_memory_ stuff is really only needed for arm64 which seems to be the
> > only
> > other one with directmap permissions. So if it could eventually have its own
> > arch_vunmap then all of the set_memory_ parts could be dropped and the
> > default
> > would just be the simple unmap then flush logic that it was originally.
> 
> I think that's probably the best solution.  If there are only two
> arches that have anything fancy here, let's just fix both of them up
> for real.
> 
> > 
> > Or we have up to three flushes for the generic version and meet the name
> > expectations and needed functionality today. I guess I'll just try that.
> > > > 
> > > > > > +        */
> > > > > > +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > > > > > +               vm_unmap_aliases();
> > > > > > +}
> > > > > > +
> > > > > >  static void __vunmap(const void *addr, int deallocate_pages)
> > > > > >  {
> > > > > >         struct vm_struct *area;
> > > > > > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int
> > > > > > deallocate_pages)
> > > > > >         debug_check_no_locks_freed(area->addr,
> > > > > > get_vm_area_size(area));
> > > > > >         debug_check_no_obj_freed(area->addr,
> > > > > > get_vm_area_size(area));
> > > > > > 
> > > > > > -       remove_vm_area(addr);
> > > > > > +       arch_vunmap(area, deallocate_pages);
> > > > > > +
> > > > > >         if (deallocate_pages) {
> > > > > >                 int i;
> > > > > > 
> > > > > > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const
> > > > > > void
> > > > > > *addr)
> > > > > >          * nother cpu's list.  schedule_work() should be fine with
> > > > > > this
> > > > > > too.
> > > > > >          */
> > > > > >         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > > > > > +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work),
> > > > > > GFP_ATOMIC);
> > > > > > +
> > > > > > +       /* If no memory for the deferred list node, give up */
> > > > > > +       if (!w)
> > > > > > +               return;
> > > > > 
> > > > > That's nasty.  I see what you're trying to do here, but I think you're
> > > > > solving a problem that doesn't need solving quite so urgently.  How
> > > > > about dropping this part and replacing it with a comment like "NB:
> > > > > this writes a word to a potentially executable address.  It would be
> > > > > nice if we could avoid doing this."  And maybe a future patch could
> > > > > more robustly avoid it without risking memory leaks.
> > > > 
> > > > Yea, sorry I should have called this out, because I wasn't sure on how
> > > > likely
> > > > that was to happen. I did find some other places in the kernel with the
> > > > same
> > > > ignoring logic.
> > > > 
> > > > I'll have to think though, I am not sure what the alternative is. Since
> > > > the
> > > > memory can be RO in the module_memfree case, the old method of re-using
> > > > the
> > > > allocation will no longer work. The list node could be stuffed on the
> > > > vm_struct,
> > > > but then the all of the spin_lock(&vmap_area_lock)'s need to be changed
> > > > to
> > > > work
> > > > with interrupts so that the struct could be looked up. Not sure of the
> > > > implications of that. Or maybe have some slow backup that resets the
> > > > permissions
> > > > and re-uses the allocation if kmalloc fails?
> > > > 
> > > > I guess it could also go back to the old v1 implementation that doesn't
> > > > handle
> > > > RO and the directmap, and leave the W^X violation window during
> > > > teardown.
> > > > Then
> > > > solve that problem when modules are loaded via something like Nadav's
> > > > stuff.
> > > > 
> > > 
> > > Hmm.  Switching to spin_lock_irqsave() doesn't seem so bad to me.
> > 
> > Ok.
> 
> Actually, I think I have a better solution.  Just declare the
> problematic case to be illegal: say that you may not free memory with
> the new flags set while IRQs are off.  Enforce this with a VM_WARN_ON
> in the code that reads the vfree_deferred list.

Thanks. Yea just making a rule for the one case seems better that disabling
interrupts all over. It turned out to lock in quite a few places, including the
longish lazy purge operation. Reading a little history on the deferred free list
- 6 years ago vfree used to not support interrupts at all, and different clients
had their own work queues. So this will just be having the original situation 
for the new vm flag.

I think we only need to move the module init section free from the RCU callback
to a work queue, to get to the point where, functionally wise, everything should
work with the existing deferred free list implementation (since then we will
just not use it for the new special memory case). We could also just add a
WARN_ON(in_interrupt()) in module_memfree to give context to some callers to
what will soon be a "BUG: unable to handle kernel paging request". Any objection
to leaving it there?

Over the above, moving the vfree_deferred list to the struct and the associated
cost of the lookup in every interrupt/atomic vfree would enable a WARN and the
handling of a (declared) illegal case that could deadlock anyway, right? Is it
worth it?

I'm not sure we why we wouldn't have deadlocks in normal interrupt vfrees if we
don't use irq spinlocks everywhere...I may be missing your insight.

Andy Lutomirski Dec. 18, 2018, 1:02 a.m. UTC | #7

On Mon, Dec 17, 2018 at 4:24 PM Edgecombe, Rick P
<[e-mail]> wrote:
>
> On Sat, 2018-12-15 at 10:52 -0800, Andy Lutomirski wrote:
> > On Wed, Dec 12, 2018 at 2:01 PM Edgecombe, Rick P
> > <[e-mail]> wrote:
> > >
> > > On Wed, 2018-12-12 at 11:57 -0800, Andy Lutomirski wrote:
> > > > On Wed, Dec 12, 2018 at 11:50 AM Edgecombe, Rick P
> > > > <[e-mail]> wrote:
> > > > >
> > > > > On Tue, 2018-12-11 at 18:20 -0800, Andy Lutomirski wrote:
> > > > > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > > > > <[e-mail]> wrote:
> > > > > > >
> > > > > > > This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS,
> > > > > > > for
> > > > > > > enabling vfree operations to immediately clear executable TLB
> > > > > > > entries to
> > > > > > > freed
> > > > > > > pages, and handle freeing memory with special permissions.
> > > > > > >
> > > > > > > In order to support vfree being called on memory that might be RO,
> > > > > > > the
> > > > > > > vfree
> > > > > > > deferred list node is moved to a kmalloc allocated struct, from
> > > > > > > where it
> > > > > > > is
> > > > > > > today, reusing the allocation being freed.
> > > > > > >
> > > > > > > arch_vunmap is a new __weak function that implements the actual
> > > > > > > unmapping
> > > > > > > and
> > > > > > > resetting of the direct map permissions. It can be overridden by
> > > > > > > more
> > > > > > > efficient
> > > > > > > architecture specific implementations.
> > > > > > >
> > > > > > > For the default implementation, it uses architecture agnostic
> > > > > > > methods
> > > > > > > which
> > > > > > > are
> > > > > > > equivalent to what most usages do before calling vfree. So now it is
> > > > > > > just
> > > > > > > centralized here.
> > > > > > >
> > > > > > > This implementation derives from two sketches from Dave Hansen and
> > > > > > > Andy
> > > > > > > Lutomirski.
> > > > > > >
> > > > > > > Suggested-by: Dave Hansen <[e-mail]>
> > > > > > > Suggested-by: Andy Lutomirski <[e-mail]>
> > > > > > > Suggested-by: Will Deacon <[e-mail]>
> > > > > > > Signed-off-by: Rick Edgecombe <[e-mail]>
> > > > > > > ---
> > > > > > >  include/linux/vmalloc.h |  2 ++
> > > > > > >  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++-
> > > > > > > ---
> > > > > > >  2 files changed, 69 insertions(+), 6 deletions(-)
> > > > > > >
> > > > > > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > > > > > > index 398e9c95cd61..872bcde17aca 100644
> > > > > > > --- a/include/linux/vmalloc.h
> > > > > > > +++ b/include/linux/vmalloc.h
> > > > > > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in
> > > > > > > notifier.h
> > > > > > > */
> > > > > > >  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not
> > > > > > > fully
> > > > > > > initialized */
> > > > > > >  #define VM_NO_GUARD            0x00000040      /* don't add guard
> > > > > > > page
> > > > > > > */
> > > > > > >  #define VM_KASAN               0x00000080      /* has allocated
> > > > > > > kasan
> > > > > > > shadow memory */
> > > > > > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before
> > > > > > > releasing
> > > > > > > pages */
> > > > > > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with
> > > > > > > special
> > > > > > > perms */
> > > > > > >  /* bits [20..32] reserved for arch specific ioremap internals */
> > > > > > >
> > > > > > >  /*
> > > > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > > > index 97d4b25d0373..02b284d2245a 100644
> > > > > > > --- a/mm/vmalloc.c
> > > > > > > +++ b/mm/vmalloc.c
> > > > > > > @@ -18,6 +18,7 @@
> > > > > > >  #include <linux/interrupt.h>
> > > > > > >  #include <linux/proc_fs.h>
> > > > > > >  #include <linux/seq_file.h>
> > > > > > > +#include <linux/set_memory.h>
> > > > > > >  #include <linux/debugobjects.h>
> > > > > > >  #include <linux/kallsyms.h>
> > > > > > >  #include <linux/list.h>
> > > > > > > @@ -38,6 +39,11 @@
> > > > > > >
> > > > > > >  #include "internal.h"
> > > > > > >
> > > > > > > +struct vfree_work {
> > > > > > > +       struct llist_node node;
> > > > > > > +       void *addr;
> > > > > > > +};
> > > > > > > +
> > > > > > >  struct vfree_deferred {
> > > > > > >         struct llist_head list;
> > > > > > >         struct work_struct wq;
> > > > > > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> > > > > > >  {
> > > > > > >         struct vfree_deferred *p = container_of(w, struct
> > > > > > > vfree_deferred,
> > > > > > > wq);
> > > > > > >         struct llist_node *t, *llnode;
> > > > > > > +       struct vfree_work *cur;
> > > > > > >
> > > > > > > -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> > > > > > > -               __vunmap((void *)llnode, 1);
> > > > > > > +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> > > > > > > +               cur = container_of(llnode, struct vfree_work, node);
> > > > > > > +               __vunmap(cur->addr, 1);
> > > > > > > +               kfree(cur);
> > > > > > > +       }
> > > > > > >  }
> > > > > > >
> > > > > > >  /*** Page table manipulation functions ***/
> > > > > > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void
> > > > > > > *addr)
> > > > > > >         return NULL;
> > > > > > >  }
> > > > > > >
> > > > > > > +/*
> > > > > > > + * This function handles unmapping and resetting the direct map as
> > > > > > > efficiently
> > > > > > > + * as it can with cross arch functions. The three categories of
> > > > > > > architectures
> > > > > > > + * are:
> > > > > > > + *   1. Architectures with no set_memory implementations and no
> > > > > > > direct
> > > > > > > map
> > > > > > > + *      permissions.
> > > > > > > + *   2. Architectures with set_memory implementations but no direct
> > > > > > > map
> > > > > > > + *      permissions
> > > > > > > + *   3. Architectures with set_memory implementations and direct
> > > > > > > map
> > > > > > > permissions
> > > > > > > + */
> > > > > > > +void __weak arch_vunmap(struct vm_struct *area, int
> > > > > > > deallocate_pages)
> > > > > >
> > > > > > My general preference is to avoid __weak functions -- they don't
> > > > > > optimize well.  Instead, I prefer either:
> > > > > >
> > > > > > #ifndef arch_vunmap
> > > > > > void arch_vunmap(...);
> > > > > > #endif
> > > > > >
> > > > > > or
> > > > > >
> > > > > > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > > > > > ...
> > > > > > #endif
> > > > >
> > > > > Ok.
> > > > > >
> > > > > > > +{
> > > > > > > +       unsigned long addr = (unsigned long)area->addr;
> > > > > > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > > > > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > > > > > +
> > > > > > > +       /*
> > > > > > > +        * In case of 2 and 3, use this general way of resetting the
> > > > > > > permissions
> > > > > > > +        * on the directmap. Do NX before RW, in case of X, so there
> > > > > > > is
> > > > > > > no
> > > > > > > W^X
> > > > > > > +        * violation window.
> > > > > > > +        *
> > > > > > > +        * For case 1 these will be noops.
> > > > > > > +        */
> > > > > > > +       if (immediate)
> > > > > > > +               set_memory_nx(addr, area->nr_pages);
> > > > > > > +       if (deallocate_pages && special)
> > > > > > > +               set_memory_rw(addr, area->nr_pages);
> > > > > >
> > > > > > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> > > > > > want that alias gone before any deallocation happens".
> > > > > > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> > > > > > me, please".  deallocate means "this was vfree -- please free the
> > > > > > pages".  I'm not convinced that all the various combinations make
> > > > > > sense.  Do we really need both flags?
> > > > >
> > > > > VM_HAS_SPECIAL_PERMS is supposed to mean, like you said, "reset the
> > > > > direct
> > > > > map".
> > > > > Where VM_IMMEDIATE_UNMAP means, the vmalloc allocation has extra
> > > > > capabilties
> > > > > where we don't want to leave an enhanced capability TLB entry to the
> > > > > freed
> > > > > page.
> > > > >
> > > > > I was trying to pick names that could apply more generally for potential
> > > > > future
> > > > > special memory capabilities. Today VM_HAS_SPECIAL_PERMS does just mean
> > > > > reset
> > > > > write to the directmap and VM_IMMEDIATE_UNMAP means vmalloc mapping is
> > > > > executable.
> > > > >
> > > > > A present day reason for keeping both flags is, it is more efficient in
> > > > > the
> > > > > arch-agnostic implementation when freeing memory that is just RO and not
> > > > > executable. It saves a TLB flush.
> > > > >
> > > > > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> > > > > > not immediate.)
> > > > >
> > > > > True, maybe VM_MUST_FLUSH or something else?
> > > > >
> > > > > > If we do keep both flags, maybe some restructuring would make sense,
> > > > > > like this, perhaps.  Sorry about horrible whitespace damage.
> > > > > >
> > > > > > if (special) {
> > > > > >   /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages.
> > > > > > */
> > > > > >   WARN_ON_ONCE(!deallocate_pages);
> > > > > >
> > > > > >   if (immediate) {
> > > > > >     /* It's possible that the vmap alias is X and we're about to make
> > > > > > the direct map RW.  To avoid a window where executable memory is
> > > > > > writable, first mark the vmap alias NX.  This is silly, since we're
> > > > > > about to *unmap* it, but this is the best we can do if all we have to
> > > > > > work with is the set_memory_abc() APIs.  Architectures should override
> > > > > > this whole function to get better behavior. */
> > > > > >     set_memory_nx(...);
> > > > > >   }
> > > > > >
> > > > > >   set_memory_rw(addr, area->nr_pages);
> > > > > > }
> > > > >
> > > > > Ok.
> > > > >
> > > > > >
> > > > > > > +
> > > > > > > +       /* Always actually remove the area */
> > > > > > > +       remove_vm_area(area->addr);
> > > > > > > +
> > > > > > > +       /*
> > > > > > > +        * Need to flush the TLB before freeing pages in the case of
> > > > > > > this
> > > > > > > flag.
> > > > > > > +        * As long as that's happening, unmap aliases.
> > > > > > > +        *
> > > > > > > +        * For 2 and 3, this will not be needed because of the
> > > > > > > set_memory_nx
> > > > > > > +        * above, because the stale TLBs will be NX.
> > > > > >
> > > > > > I'm not sure I agree with this comment.  If the caller asked for an
> > > > > > immediate unmap, we should give an immediate unmap.  But I'm still not
> > > > > > sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
> > > > >
> > > > > Yea. I was just trying to save a TLB flush, since for today's callers
> > > > > that
> > > > > have
> > > > > set_memory there isn't a security downside I know of to just leaving it
> > > > > NX.
> > > > > Maybe its not worth the tradeoff of confusion? Or I can clarify that in
> > > > > the
> > > > > comment.
> > > >
> > > > Don't both of the users in your series set both flags, though?  My
> > > > real objection to having them be separate is that, in the absence of
> > > > users, it's less clear exactly what they should do and the code
> > > > doesn't get exercised.
> > >
> > > The only "just RO" user today is one of the BPF allocations. I don't have a
> > > strong objection to combining them, just explaining the thinking. I guess if
> > > we
> > > could always add another flag later if it becomes more needed.
> > >
> > > > If you document that VM_IMMEDIATE_UNMAP means "I want the TLB entries
> > > > gone", then I can re-review the code in light of that.  But then I'm
> > > > unconvinced by your generic implementation, since set_memory_nx()
> > > > seems like an odd way to go about it.
> > >
> > > Masami Hiramatsu pointed out if we don't do set_memory_nx before
> > > set_memory_rw,
> > > then there will be a small window of W^X violation. So that was the concern
> > > for
> > > the executable case, regardless of the semantics. I think the concern
> > > applies
> > > for any "special capability" permissions. Alternatively, if we
> > > remove_vm_area
> > > before we reset the direct map perms RW, maybe that would accomplish the
> > > same
> > > thing, if that's possible in a cross arch way. Maybe this is too much
> > > designing
> > > for hypothetical future... just was trying to avoid having to change the
> > > interface, and could just update the generic implementation if new
> > > permissions
> > > or usages come up.
> > >
> > > The set_memory_ stuff is really only needed for arm64 which seems to be the
> > > only
> > > other one with directmap permissions. So if it could eventually have its own
> > > arch_vunmap then all of the set_memory_ parts could be dropped and the
> > > default
> > > would just be the simple unmap then flush logic that it was originally.
> >
> > I think that's probably the best solution.  If there are only two
> > arches that have anything fancy here, let's just fix both of them up
> > for real.
> >
> > >
> > > Or we have up to three flushes for the generic version and meet the name
> > > expectations and needed functionality today. I guess I'll just try that.
> > > > >
> > > > > > > +        */
> > > > > > > +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > > > > > > +               vm_unmap_aliases();
> > > > > > > +}
> > > > > > > +
> > > > > > >  static void __vunmap(const void *addr, int deallocate_pages)
> > > > > > >  {
> > > > > > >         struct vm_struct *area;
> > > > > > > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int
> > > > > > > deallocate_pages)
> > > > > > >         debug_check_no_locks_freed(area->addr,
> > > > > > > get_vm_area_size(area));
> > > > > > >         debug_check_no_obj_freed(area->addr,
> > > > > > > get_vm_area_size(area));
> > > > > > >
> > > > > > > -       remove_vm_area(addr);
> > > > > > > +       arch_vunmap(area, deallocate_pages);
> > > > > > > +
> > > > > > >         if (deallocate_pages) {
> > > > > > >                 int i;
> > > > > > >
> > > > > > > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const
> > > > > > > void
> > > > > > > *addr)
> > > > > > >          * nother cpu's list.  schedule_work() should be fine with
> > > > > > > this
> > > > > > > too.
> > > > > > >          */
> > > > > > >         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > > > > > > +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work),
> > > > > > > GFP_ATOMIC);
> > > > > > > +
> > > > > > > +       /* If no memory for the deferred list node, give up */
> > > > > > > +       if (!w)
> > > > > > > +               return;
> > > > > >
> > > > > > That's nasty.  I see what you're trying to do here, but I think you're
> > > > > > solving a problem that doesn't need solving quite so urgently.  How
> > > > > > about dropping this part and replacing it with a comment like "NB:
> > > > > > this writes a word to a potentially executable address.  It would be
> > > > > > nice if we could avoid doing this."  And maybe a future patch could
> > > > > > more robustly avoid it without risking memory leaks.
> > > > >
> > > > > Yea, sorry I should have called this out, because I wasn't sure on how
> > > > > likely
> > > > > that was to happen. I did find some other places in the kernel with the
> > > > > same
> > > > > ignoring logic.
> > > > >
> > > > > I'll have to think though, I am not sure what the alternative is. Since
> > > > > the
> > > > > memory can be RO in the module_memfree case, the old method of re-using
> > > > > the
> > > > > allocation will no longer work. The list node could be stuffed on the
> > > > > vm_struct,
> > > > > but then the all of the spin_lock(&vmap_area_lock)'s need to be changed
> > > > > to
> > > > > work
> > > > > with interrupts so that the struct could be looked up. Not sure of the
> > > > > implications of that. Or maybe have some slow backup that resets the
> > > > > permissions
> > > > > and re-uses the allocation if kmalloc fails?
> > > > >
> > > > > I guess it could also go back to the old v1 implementation that doesn't
> > > > > handle
> > > > > RO and the directmap, and leave the W^X violation window during
> > > > > teardown.
> > > > > Then
> > > > > solve that problem when modules are loaded via something like Nadav's
> > > > > stuff.
> > > > >
> > > >
> > > > Hmm.  Switching to spin_lock_irqsave() doesn't seem so bad to me.
> > >
> > > Ok.
> >
> > Actually, I think I have a better solution.  Just declare the
> > problematic case to be illegal: say that you may not free memory with
> > the new flags set while IRQs are off.  Enforce this with a VM_WARN_ON
> > in the code that reads the vfree_deferred list.
>
> Thanks. Yea just making a rule for the one case seems better that disabling
> interrupts all over. It turned out to lock in quite a few places, including the
> longish lazy purge operation. Reading a little history on the deferred free list
> - 6 years ago vfree used to not support interrupts at all, and different clients
> had their own work queues. So this will just be having the original situation
> for the new vm flag.
>
> I think we only need to move the module init section free from the RCU callback
> to a work queue, to get to the point where, functionally wise, everything should
> work with the existing deferred free list implementation (since then we will
> just not use it for the new special memory case).

> We could also just add a
> WARN_ON(in_interrupt()) in module_memfree to give context to some callers to
> what will soon be a "BUG: unable to handle kernel paging request". Any objection
> to leaving it there?
>

Seems reasonable.

> Over the above, moving the vfree_deferred list to the struct and the associated
> cost of the lookup in every interrupt/atomic vfree would enable a WARN and the
> handling of a (declared) illegal case that could deadlock anyway, right? Is it
> worth it?

I suspect it's not worth it.

>
> I'm not sure we why we wouldn't have deadlocks in normal interrupt vfrees if we
> don't use irq spinlocks everywhere...I may be missing your insight.
>

I may be misunderstanding your question, but: I suspect that we can
easily deadlock if vfree() is called with IRQs off but
!in_interrupt().  Perhaps no one does that?  At the very least, I
assume that lockdep would scream loudly if this happened.

--Andy

Ard Biesheuvel Dec. 21, 2018, 4:39 p.m. UTC | #8

On Wed, 12 Dec 2018 at 03:20, Andy Lutomirski <[e-mail]> wrote:
>
> On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> <[e-mail]> wrote:
> >
> > This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
> > enabling vfree operations to immediately clear executable TLB entries to freed
> > pages, and handle freeing memory with special permissions.
> >
> > In order to support vfree being called on memory that might be RO, the vfree
> > deferred list node is moved to a kmalloc allocated struct, from where it is
> > today, reusing the allocation being freed.
> >
> > arch_vunmap is a new __weak function that implements the actual unmapping and
> > resetting of the direct map permissions. It can be overridden by more efficient
> > architecture specific implementations.
> >
> > For the default implementation, it uses architecture agnostic methods which are
> > equivalent to what most usages do before calling vfree. So now it is just
> > centralized here.
> >
> > This implementation derives from two sketches from Dave Hansen and Andy
> > Lutomirski.
> >
> > Suggested-by: Dave Hansen <[e-mail]>
> > Suggested-by: Andy Lutomirski <[e-mail]>
> > Suggested-by: Will Deacon <[e-mail]>
> > Signed-off-by: Rick Edgecombe <[e-mail]>
> > ---
> >  include/linux/vmalloc.h |  2 ++
> >  mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
> >  2 files changed, 69 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > index 398e9c95cd61..872bcde17aca 100644
> > --- a/include/linux/vmalloc.h
> > +++ b/include/linux/vmalloc.h
> > @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h */
> >  #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not fully initialized */
> >  #define VM_NO_GUARD            0x00000040      /* don't add guard page */
> >  #define VM_KASAN               0x00000080      /* has allocated kasan shadow memory */
> > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before releasing pages */
> > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with special perms */
> >  /* bits [20..32] reserved for arch specific ioremap internals */
> >
> >  /*
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 97d4b25d0373..02b284d2245a 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/interrupt.h>
> >  #include <linux/proc_fs.h>
> >  #include <linux/seq_file.h>
> > +#include <linux/set_memory.h>
> >  #include <linux/debugobjects.h>
> >  #include <linux/kallsyms.h>
> >  #include <linux/list.h>
> > @@ -38,6 +39,11 @@
> >
> >  #include "internal.h"
> >
> > +struct vfree_work {
> > +       struct llist_node node;
> > +       void *addr;
> > +};
> > +
> >  struct vfree_deferred {
> >         struct llist_head list;
> >         struct work_struct wq;
> > @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> >  {
> >         struct vfree_deferred *p = container_of(w, struct vfree_deferred, wq);
> >         struct llist_node *t, *llnode;
> > +       struct vfree_work *cur;
> >
> > -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> > -               __vunmap((void *)llnode, 1);
> > +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> > +               cur = container_of(llnode, struct vfree_work, node);
> > +               __vunmap(cur->addr, 1);
> > +               kfree(cur);
> > +       }
> >  }
> >
> >  /*** Page table manipulation functions ***/
> > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void *addr)
> >         return NULL;
> >  }
> >
> > +/*
> > + * This function handles unmapping and resetting the direct map as efficiently
> > + * as it can with cross arch functions. The three categories of architectures
> > + * are:
> > + *   1. Architectures with no set_memory implementations and no direct map
> > + *      permissions.
> > + *   2. Architectures with set_memory implementations but no direct map
> > + *      permissions
> > + *   3. Architectures with set_memory implementations and direct map permissions
> > + */
> > +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
>
> My general preference is to avoid __weak functions -- they don't
> optimize well.  Instead, I prefer either:
>
> #ifndef arch_vunmap
> void arch_vunmap(...);
> #endif
>
> or
>
> #ifdef CONFIG_HAVE_ARCH_VUNMAP
> ...
> #endif
>
>
> > +{
> > +       unsigned long addr = (unsigned long)area->addr;
> > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > +
> > +       /*
> > +        * In case of 2 and 3, use this general way of resetting the permissions
> > +        * on the directmap. Do NX before RW, in case of X, so there is no W^X
> > +        * violation window.
> > +        *
> > +        * For case 1 these will be noops.
> > +        */
> > +       if (immediate)
> > +               set_memory_nx(addr, area->nr_pages);
> > +       if (deallocate_pages && special)
> > +               set_memory_rw(addr, area->nr_pages);
>
> Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> want that alias gone before any deallocation happens".
> VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> me, please".  deallocate means "this was vfree -- please free the
> pages".  I'm not convinced that all the various combinations make
> sense.  Do we really need both flags?
>
> (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> not immediate.)
>
> If we do keep both flags, maybe some restructuring would make sense,
> like this, perhaps.  Sorry about horrible whitespace damage.
>
> if (special) {
>   /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
>   WARN_ON_ONCE(!deallocate_pages);
>
>   if (immediate) {
>     /* It's possible that the vmap alias is X and we're about to make
> the direct map RW.  To avoid a window where executable memory is
> writable, first mark the vmap alias NX.  This is silly, since we're
> about to *unmap* it, but this is the best we can do if all we have to
> work with is the set_memory_abc() APIs.  Architectures should override
> this whole function to get better behavior. */

So can't we fix this first? Assuming that architectures that bother to
implement them will not have executable mappings in the linear region,
all we'd need is set_linear_range_ro/rw() routines that default to
doing nothing, and encapsulate the existing code for x86 and arm64.
That way, we can handle do things in the proper order, i.e., release
the vmalloc mapping (without caring about the permissions), restore
the linear alias attributes, and finally release the pages.


>     set_memory_nx(...);
>   }
>
>   set_memory_rw(addr, area->nr_pages);
> }
>
>
> > +
> > +       /* Always actually remove the area */
> > +       remove_vm_area(area->addr);
> > +
> > +       /*
> > +        * Need to flush the TLB before freeing pages in the case of this flag.
> > +        * As long as that's happening, unmap aliases.
> > +        *
> > +        * For 2 and 3, this will not be needed because of the set_memory_nx
> > +        * above, because the stale TLBs will be NX.
>
> I'm not sure I agree with this comment.  If the caller asked for an
> immediate unmap, we should give an immediate unmap.  But I'm still not
> sure I see why VM_IMMEDIATE_UNMAP needs to exist as a separate flag.
>
> > +        */
> > +       if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
> > +               vm_unmap_aliases();
> > +}
> > +
> >  static void __vunmap(const void *addr, int deallocate_pages)
> >  {
> >         struct vm_struct *area;
> > @@ -1515,7 +1567,8 @@ static void __vunmap(const void *addr, int deallocate_pages)
> >         debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> >         debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
> >
> > -       remove_vm_area(addr);
> > +       arch_vunmap(area, deallocate_pages);
> > +
> >         if (deallocate_pages) {
> >                 int i;
> >
> > @@ -1542,8 +1595,15 @@ static inline void __vfree_deferred(const void *addr)
> >          * nother cpu's list.  schedule_work() should be fine with this too.
> >          */
> >         struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
> > +       struct vfree_work *w = kmalloc(sizeof(struct vfree_work), GFP_ATOMIC);
> > +
> > +       /* If no memory for the deferred list node, give up */
> > +       if (!w)
> > +               return;
>
> That's nasty.  I see what you're trying to do here, but I think you're
> solving a problem that doesn't need solving quite so urgently.  How
> about dropping this part and replacing it with a comment like "NB:
> this writes a word to a potentially executable address.  It would be
> nice if we could avoid doing this."  And maybe a future patch could
> more robustly avoid it without risking memory leaks.

Andy Lutomirski Dec. 21, 2018, 5:12 p.m. UTC | #9

> On Dec 21, 2018, at 9:39 AM, Ard Biesheuvel <[e-mail]> wrote:
>
>> On Wed, 12 Dec 2018 at 03:20, Andy Lutomirski <[e-mail]> wrote:
>>
>> On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
>> <[e-mail]> wrote:
>>>
>>> This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
>>> enabling vfree operations to immediately clear executable TLB entries to freed
>>> pages, and handle freeing memory with special permissions.
>>>
>>> In order to support vfree being called on memory that might be RO, the vfree
>>> deferred list node is moved to a kmalloc allocated struct, from where it is
>>> today, reusing the allocation being freed.
>>>
>>> arch_vunmap is a new __weak function that implements the actual unmapping and
>>> resetting of the direct map permissions. It can be overridden by more efficient
>>> architecture specific implementations.
>>>
>>> For the default implementation, it uses architecture agnostic methods which are
>>> equivalent to what most usages do before calling vfree. So now it is just
>>> centralized here.
>>>
>>> This implementation derives from two sketches from Dave Hansen and Andy
>>> Lutomirski.
>>>
>>> Suggested-by: Dave Hansen <[e-mail]>
>>> Suggested-by: Andy Lutomirski <[e-mail]>
>>> Suggested-by: Will Deacon <[e-mail]>
>>> Signed-off-by: Rick Edgecombe <[e-mail]>
>>> ---
>>> include/linux/vmalloc.h |  2 ++
>>> mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
>>> 2 files changed, 69 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
>>> index 398e9c95cd61..872bcde17aca 100644
>>> --- a/include/linux/vmalloc.h
>>> +++ b/include/linux/vmalloc.h
>>> @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h */
>>> #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not fully initialized */
>>> #define VM_NO_GUARD            0x00000040      /* don't add guard page */
>>> #define VM_KASAN               0x00000080      /* has allocated kasan shadow memory */
>>> +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before releasing pages */
>>> +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with special perms */
>>> /* bits [20..32] reserved for arch specific ioremap internals */
>>>
>>> /*
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index 97d4b25d0373..02b284d2245a 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -18,6 +18,7 @@
>>> #include <linux/interrupt.h>
>>> #include <linux/proc_fs.h>
>>> #include <linux/seq_file.h>
>>> +#include <linux/set_memory.h>
>>> #include <linux/debugobjects.h>
>>> #include <linux/kallsyms.h>
>>> #include <linux/list.h>
>>> @@ -38,6 +39,11 @@
>>>
>>> #include "internal.h"
>>>
>>> +struct vfree_work {
>>> +       struct llist_node node;
>>> +       void *addr;
>>> +};
>>> +
>>> struct vfree_deferred {
>>>        struct llist_head list;
>>>        struct work_struct wq;
>>> @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
>>> {
>>>        struct vfree_deferred *p = container_of(w, struct vfree_deferred, wq);
>>>        struct llist_node *t, *llnode;
>>> +       struct vfree_work *cur;
>>>
>>> -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
>>> -               __vunmap((void *)llnode, 1);
>>> +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
>>> +               cur = container_of(llnode, struct vfree_work, node);
>>> +               __vunmap(cur->addr, 1);
>>> +               kfree(cur);
>>> +       }
>>> }
>>>
>>> /*** Page table manipulation functions ***/
>>> @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void *addr)
>>>        return NULL;
>>> }
>>>
>>> +/*
>>> + * This function handles unmapping and resetting the direct map as efficiently
>>> + * as it can with cross arch functions. The three categories of architectures
>>> + * are:
>>> + *   1. Architectures with no set_memory implementations and no direct map
>>> + *      permissions.
>>> + *   2. Architectures with set_memory implementations but no direct map
>>> + *      permissions
>>> + *   3. Architectures with set_memory implementations and direct map permissions
>>> + */
>>> +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
>>
>> My general preference is to avoid __weak functions -- they don't
>> optimize well.  Instead, I prefer either:
>>
>> #ifndef arch_vunmap
>> void arch_vunmap(...);
>> #endif
>>
>> or
>>
>> #ifdef CONFIG_HAVE_ARCH_VUNMAP
>> ...
>> #endif
>>
>>
>>> +{
>>> +       unsigned long addr = (unsigned long)area->addr;
>>> +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
>>> +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
>>> +
>>> +       /*
>>> +        * In case of 2 and 3, use this general way of resetting the permissions
>>> +        * on the directmap. Do NX before RW, in case of X, so there is no W^X
>>> +        * violation window.
>>> +        *
>>> +        * For case 1 these will be noops.
>>> +        */
>>> +       if (immediate)
>>> +               set_memory_nx(addr, area->nr_pages);
>>> +       if (deallocate_pages && special)
>>> +               set_memory_rw(addr, area->nr_pages);
>>
>> Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
>> want that alias gone before any deallocation happens".
>> VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
>> me, please".  deallocate means "this was vfree -- please free the
>> pages".  I'm not convinced that all the various combinations make
>> sense.  Do we really need both flags?
>>
>> (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
>> not immediate.)
>>
>> If we do keep both flags, maybe some restructuring would make sense,
>> like this, perhaps.  Sorry about horrible whitespace damage.
>>
>> if (special) {
>>  /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
>>  WARN_ON_ONCE(!deallocate_pages);
>>
>>  if (immediate) {
>>    /* It's possible that the vmap alias is X and we're about to make
>> the direct map RW.  To avoid a window where executable memory is
>> writable, first mark the vmap alias NX.  This is silly, since we're
>> about to *unmap* it, but this is the best we can do if all we have to
>> work with is the set_memory_abc() APIs.  Architectures should override
>> this whole function to get better behavior. */
>
> So can't we fix this first? Assuming that architectures that bother to
> implement them will not have executable mappings in the linear region,
> all we'd need is set_linear_range_ro/rw() routines that default to
> doing nothing, and encapsulate the existing code for x86 and arm64.
> That way, we can handle do things in the proper order, i.e., release
> the vmalloc mapping (without caring about the permissions), restore
> the linear alias attributes, and finally release the pages.

Seems reasonable, except that I think it should be
set_linear_range_not_present() and set_linear_range_rw(), for three
reasons:

1. It’s not at all clear to me that we need to keep the linear mapping
around for modules.

2. At least on x86, the obvious algorithm to do the free operation
with a single flush requires it.  Someone should probably confirm that
arm’s TLB works the same way, i.e. that no flush is needed when
changing from not-present (or whatever ARM calls it) to RW.

3. Anyone playing with XPFO wants this facility anyway.  In fact, with
this change, Rick’s series will more or less implement XPFO for
vmalloc memory :)

Does that seem reasonable to you?

Ard Biesheuvel Dec. 21, 2018, 5:25 p.m. UTC | #10

On Fri, 21 Dec 2018 at 18:12, Andy Lutomirski <[e-mail]> wrote:
>
> > On Dec 21, 2018, at 9:39 AM, Ard Biesheuvel <[e-mail]> wrote:
> >
> >> On Wed, 12 Dec 2018 at 03:20, Andy Lutomirski <[e-mail]> wrote:
> >>
> >> On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> >> <[e-mail]> wrote:
> >>>
> >>> This adds two new flags VM_IMMEDIATE_UNMAP and VM_HAS_SPECIAL_PERMS, for
> >>> enabling vfree operations to immediately clear executable TLB entries to freed
> >>> pages, and handle freeing memory with special permissions.
> >>>
> >>> In order to support vfree being called on memory that might be RO, the vfree
> >>> deferred list node is moved to a kmalloc allocated struct, from where it is
> >>> today, reusing the allocation being freed.
> >>>
> >>> arch_vunmap is a new __weak function that implements the actual unmapping and
> >>> resetting of the direct map permissions. It can be overridden by more efficient
> >>> architecture specific implementations.
> >>>
> >>> For the default implementation, it uses architecture agnostic methods which are
> >>> equivalent to what most usages do before calling vfree. So now it is just
> >>> centralized here.
> >>>
> >>> This implementation derives from two sketches from Dave Hansen and Andy
> >>> Lutomirski.
> >>>
> >>> Suggested-by: Dave Hansen <[e-mail]>
> >>> Suggested-by: Andy Lutomirski <[e-mail]>
> >>> Suggested-by: Will Deacon <[e-mail]>
> >>> Signed-off-by: Rick Edgecombe <[e-mail]>
> >>> ---
> >>> include/linux/vmalloc.h |  2 ++
> >>> mm/vmalloc.c            | 73 +++++++++++++++++++++++++++++++++++++----
> >>> 2 files changed, 69 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> >>> index 398e9c95cd61..872bcde17aca 100644
> >>> --- a/include/linux/vmalloc.h
> >>> +++ b/include/linux/vmalloc.h
> >>> @@ -21,6 +21,8 @@ struct notifier_block;                /* in notifier.h */
> >>> #define VM_UNINITIALIZED       0x00000020      /* vm_struct is not fully initialized */
> >>> #define VM_NO_GUARD            0x00000040      /* don't add guard page */
> >>> #define VM_KASAN               0x00000080      /* has allocated kasan shadow memory */
> >>> +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush before releasing pages */
> >>> +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be freed with special perms */
> >>> /* bits [20..32] reserved for arch specific ioremap internals */
> >>>
> >>> /*
> >>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> >>> index 97d4b25d0373..02b284d2245a 100644
> >>> --- a/mm/vmalloc.c
> >>> +++ b/mm/vmalloc.c
> >>> @@ -18,6 +18,7 @@
> >>> #include <linux/interrupt.h>
> >>> #include <linux/proc_fs.h>
> >>> #include <linux/seq_file.h>
> >>> +#include <linux/set_memory.h>
> >>> #include <linux/debugobjects.h>
> >>> #include <linux/kallsyms.h>
> >>> #include <linux/list.h>
> >>> @@ -38,6 +39,11 @@
> >>>
> >>> #include "internal.h"
> >>>
> >>> +struct vfree_work {
> >>> +       struct llist_node node;
> >>> +       void *addr;
> >>> +};
> >>> +
> >>> struct vfree_deferred {
> >>>        struct llist_head list;
> >>>        struct work_struct wq;
> >>> @@ -50,9 +56,13 @@ static void free_work(struct work_struct *w)
> >>> {
> >>>        struct vfree_deferred *p = container_of(w, struct vfree_deferred, wq);
> >>>        struct llist_node *t, *llnode;
> >>> +       struct vfree_work *cur;
> >>>
> >>> -       llist_for_each_safe(llnode, t, llist_del_all(&p->list))
> >>> -               __vunmap((void *)llnode, 1);
> >>> +       llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
> >>> +               cur = container_of(llnode, struct vfree_work, node);
> >>> +               __vunmap(cur->addr, 1);
> >>> +               kfree(cur);
> >>> +       }
> >>> }
> >>>
> >>> /*** Page table manipulation functions ***/
> >>> @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const void *addr)
> >>>        return NULL;
> >>> }
> >>>
> >>> +/*
> >>> + * This function handles unmapping and resetting the direct map as efficiently
> >>> + * as it can with cross arch functions. The three categories of architectures
> >>> + * are:
> >>> + *   1. Architectures with no set_memory implementations and no direct map
> >>> + *      permissions.
> >>> + *   2. Architectures with set_memory implementations but no direct map
> >>> + *      permissions
> >>> + *   3. Architectures with set_memory implementations and direct map permissions
> >>> + */
> >>> +void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
> >>
> >> My general preference is to avoid __weak functions -- they don't
> >> optimize well.  Instead, I prefer either:
> >>
> >> #ifndef arch_vunmap
> >> void arch_vunmap(...);
> >> #endif
> >>
> >> or
> >>
> >> #ifdef CONFIG_HAVE_ARCH_VUNMAP
> >> ...
> >> #endif
> >>
> >>
> >>> +{
> >>> +       unsigned long addr = (unsigned long)area->addr;
> >>> +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> >>> +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> >>> +
> >>> +       /*
> >>> +        * In case of 2 and 3, use this general way of resetting the permissions
> >>> +        * on the directmap. Do NX before RW, in case of X, so there is no W^X
> >>> +        * violation window.
> >>> +        *
> >>> +        * For case 1 these will be noops.
> >>> +        */
> >>> +       if (immediate)
> >>> +               set_memory_nx(addr, area->nr_pages);
> >>> +       if (deallocate_pages && special)
> >>> +               set_memory_rw(addr, area->nr_pages);
> >>
> >> Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means "I
> >> want that alias gone before any deallocation happens".
> >> VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix it for
> >> me, please".  deallocate means "this was vfree -- please free the
> >> pages".  I'm not convinced that all the various combinations make
> >> sense.  Do we really need both flags?
> >>
> >> (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if in_interrupt(), it's
> >> not immediate.)
> >>
> >> If we do keep both flags, maybe some restructuring would make sense,
> >> like this, perhaps.  Sorry about horrible whitespace damage.
> >>
> >> if (special) {
> >>  /* VM_HAS_SPECIAL_PERMS makes little sense without deallocate_pages. */
> >>  WARN_ON_ONCE(!deallocate_pages);
> >>
> >>  if (immediate) {
> >>    /* It's possible that the vmap alias is X and we're about to make
> >> the direct map RW.  To avoid a window where executable memory is
> >> writable, first mark the vmap alias NX.  This is silly, since we're
> >> about to *unmap* it, but this is the best we can do if all we have to
> >> work with is the set_memory_abc() APIs.  Architectures should override
> >> this whole function to get better behavior. */
> >
> > So can't we fix this first? Assuming that architectures that bother to
> > implement them will not have executable mappings in the linear region,
> > all we'd need is set_linear_range_ro/rw() routines that default to
> > doing nothing, and encapsulate the existing code for x86 and arm64.
> > That way, we can handle do things in the proper order, i.e., release
> > the vmalloc mapping (without caring about the permissions), restore
> > the linear alias attributes, and finally release the pages.
>
> Seems reasonable, except that I think it should be
> set_linear_range_not_present() and set_linear_range_rw(), for three
> reasons:
>
> 1. It’s not at all clear to me that we need to keep the linear mapping
> around for modules.
>

I'm pretty sure hibernate on arm64 will have to be fixed, since it
expects to be able to read all valid pages via the linear map. But we
can fix that.

> 2. At least on x86, the obvious algorithm to do the free operation
> with a single flush requires it.  Someone should probably confirm that
> arm’s TLB works the same way, i.e. that no flush is needed when
> changing from not-present (or whatever ARM calls it) to RW.
>

Good point. ARM is similar in this regard, although we'll probably
clear the access flag rather than unmap the page entirely (which is
treated the same way in terms of required TLB management)

> 3. Anyone playing with XPFO wants this facility anyway.  In fact, with
> this change, Rick’s series will more or less implement XPFO for
> vmalloc memory :)
>
> Does that seem reasonable to you?

Absolutely.

Rick Edgecombe Dec. 21, 2018, 7:57 p.m. UTC | #11

On Fri, 2018-12-21 at 18:25 +0100, Ard Biesheuvel wrote:
> On Fri, 21 Dec 2018 at 18:12, Andy Lutomirski <[e-mail]>
> wrote:
> > > On Dec 21, 2018, at 9:39 AM, Ard Biesheuvel <
> > > [e-mail]> wrote:
> > > 
> > > > On Wed, 12 Dec 2018 at 03:20, Andy Lutomirski <[e-mail]>
> > > > wrote:
> > > > 
> > > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > > <[e-mail]> wrote:
> > > > > This adds two new flags VM_IMMEDIATE_UNMAP and
> > > > > VM_HAS_SPECIAL_PERMS, for
> > > > > enabling vfree operations to immediately clear executable TLB
> > > > > entries to freed
> > > > > pages, and handle freeing memory with special permissions.
> > > > > 
> > > > > In order to support vfree being called on memory that might
> > > > > be RO, the vfree
> > > > > deferred list node is moved to a kmalloc allocated struct,
> > > > > from where it is
> > > > > today, reusing the allocation being freed.
> > > > > 
> > > > > arch_vunmap is a new __weak function that implements the
> > > > > actual unmapping and
> > > > > resetting of the direct map permissions. It can be overridden
> > > > > by more efficient
> > > > > architecture specific implementations.
> > > > > 
> > > > > For the default implementation, it uses architecture agnostic
> > > > > methods which are
> > > > > equivalent to what most usages do before calling vfree. So
> > > > > now it is just
> > > > > centralized here.
> > > > > 
> > > > > This implementation derives from two sketches from Dave
> > > > > Hansen and Andy
> > > > > Lutomirski.
> > > > > 
> > > > > Suggested-by: Dave Hansen <[e-mail]>
> > > > > Suggested-by: Andy Lutomirski <[e-mail]>
> > > > > Suggested-by: Will Deacon <[e-mail]>
> > > > > Signed-off-by: Rick Edgecombe <[e-mail]>
> > > > > ---
> > > > > include/linux/vmalloc.h |  2 ++
> > > > > mm/vmalloc.c            | 73
> > > > > +++++++++++++++++++++++++++++++++++++----
> > > > > 2 files changed, 69 insertions(+), 6 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/vmalloc.h
> > > > > b/include/linux/vmalloc.h
> > > > > index 398e9c95cd61..872bcde17aca 100644
> > > > > --- a/include/linux/vmalloc.h
> > > > > +++ b/include/linux/vmalloc.h
> > > > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in
> > > > > notifier.h */
> > > > > #define VM_UNINITIALIZED       0x00000020      /* vm_struct
> > > > > is not fully initialized */
> > > > > #define VM_NO_GUARD            0x00000040      /* don't add
> > > > > guard page */
> > > > > #define VM_KASAN               0x00000080      /* has
> > > > > allocated kasan shadow memory */
> > > > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush
> > > > > before releasing pages */
> > > > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be
> > > > > freed with special perms */
> > > > > /* bits [20..32] reserved for arch specific ioremap internals
> > > > > */
> > > > > 
> > > > > /*
> > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > index 97d4b25d0373..02b284d2245a 100644
> > > > > --- a/mm/vmalloc.c
> > > > > +++ b/mm/vmalloc.c
> > > > > @@ -18,6 +18,7 @@
> > > > > #include <linux/interrupt.h>
> > > > > #include <linux/proc_fs.h>
> > > > > #include <linux/seq_file.h>
> > > > > +#include <linux/set_memory.h>
> > > > > #include <linux/debugobjects.h>
> > > > > #include <linux/kallsyms.h>
> > > > > #include <linux/list.h>
> > > > > @@ -38,6 +39,11 @@
> > > > > 
> > > > > #include "internal.h"
> > > > > 
> > > > > +struct vfree_work {
> > > > > +       struct llist_node node;
> > > > > +       void *addr;
> > > > > +};
> > > > > +
> > > > > struct vfree_deferred {
> > > > >        struct llist_head list;
> > > > >        struct work_struct wq;
> > > > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct
> > > > > *w)
> > > > > {
> > > > >        struct vfree_deferred *p = container_of(w, struct
> > > > > vfree_deferred, wq);
> > > > >        struct llist_node *t, *llnode;
> > > > > +       struct vfree_work *cur;
> > > > > 
> > > > > -       llist_for_each_safe(llnode, t, llist_del_all(&p-
> > > > > >list))
> > > > > -               __vunmap((void *)llnode, 1);
> > > > > +       llist_for_each_safe(llnode, t, llist_del_all(&p-
> > > > > >list)) {
> > > > > +               cur = container_of(llnode, struct vfree_work,
> > > > > node);
> > > > > +               __vunmap(cur->addr, 1);
> > > > > +               kfree(cur);
> > > > > +       }
> > > > > }
> > > > > 
> > > > > /*** Page table manipulation functions ***/
> > > > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const
> > > > > void *addr)
> > > > >        return NULL;
> > > > > }
> > > > > 
> > > > > +/*
> > > > > + * This function handles unmapping and resetting the direct
> > > > > map as efficiently
> > > > > + * as it can with cross arch functions. The three categories
> > > > > of architectures
> > > > > + * are:
> > > > > + *   1. Architectures with no set_memory implementations and
> > > > > no direct map
> > > > > + *      permissions.
> > > > > + *   2. Architectures with set_memory implementations but no
> > > > > direct map
> > > > > + *      permissions
> > > > > + *   3. Architectures with set_memory implementations and
> > > > > direct map permissions
> > > > > + */
> > > > > +void __weak arch_vunmap(struct vm_struct *area, int
> > > > > deallocate_pages)
> > > > 
> > > > My general preference is to avoid __weak functions -- they
> > > > don't
> > > > optimize well.  Instead, I prefer either:
> > > > 
> > > > #ifndef arch_vunmap
> > > > void arch_vunmap(...);
> > > > #endif
> > > > 
> > > > or
> > > > 
> > > > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > > > ...
> > > > #endif
> > > > 
> > > > 
> > > > > +{
> > > > > +       unsigned long addr = (unsigned long)area->addr;
> > > > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > > > +
> > > > > +       /*
> > > > > +        * In case of 2 and 3, use this general way of
> > > > > resetting the permissions
> > > > > +        * on the directmap. Do NX before RW, in case of X,
> > > > > so there is no W^X
> > > > > +        * violation window.
> > > > > +        *
> > > > > +        * For case 1 these will be noops.
> > > > > +        */
> > > > > +       if (immediate)
> > > > > +               set_memory_nx(addr, area->nr_pages);
> > > > > +       if (deallocate_pages && special)
> > > > > +               set_memory_rw(addr, area->nr_pages);
> > > > 
> > > > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means
> > > > "I
> > > > want that alias gone before any deallocation happens".
> > > > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix
> > > > it for
> > > > me, please".  deallocate means "this was vfree -- please free
> > > > the
> > > > pages".  I'm not convinced that all the various combinations
> > > > make
> > > > sense.  Do we really need both flags?
> > > > 
> > > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if
> > > > in_interrupt(), it's
> > > > not immediate.)
> > > > 
> > > > If we do keep both flags, maybe some restructuring would make
> > > > sense,
> > > > like this, perhaps.  Sorry about horrible whitespace damage.
> > > > 
> > > > if (special) {
> > > >  /* VM_HAS_SPECIAL_PERMS makes little sense without
> > > > deallocate_pages. */
> > > >  WARN_ON_ONCE(!deallocate_pages);
> > > > 
> > > >  if (immediate) {
> > > >    /* It's possible that the vmap alias is X and we're about to
> > > > make
> > > > the direct map RW.  To avoid a window where executable memory
> > > > is
> > > > writable, first mark the vmap alias NX.  This is silly, since
> > > > we're
> > > > about to *unmap* it, but this is the best we can do if all we
> > > > have to
> > > > work with is the set_memory_abc() APIs.  Architectures should
> > > > override
> > > > this whole function to get better behavior. */
> > > 
> > > So can't we fix this first? Assuming that architectures that
> > > bother to
> > > implement them will not have executable mappings in the linear
> > > region,
> > > all we'd need is set_linear_range_ro/rw() routines that default
> > > to
> > > doing nothing, and encapsulate the existing code for x86 and
> > > arm64.
> > > That way, we can handle do things in the proper order, i.e.,
> > > release
> > > the vmalloc mapping (without caring about the permissions),
> > > restore
> > > the linear alias attributes, and finally release the pages.
> > 
> > Seems reasonable, except that I think it should be
> > set_linear_range_not_present() and set_linear_range_rw(), for three
> > reasons:
> > 
> > 1. It’s not at all clear to me that we need to keep the linear
> > mapping
> > around for modules.
> > 
> 
> I'm pretty sure hibernate on arm64 will have to be fixed, since it
> expects to be able to read all valid pages via the linear map. But we
> can fix that.
Hmm, now I wonder what else might be trying to access the entire direct
map for some reason. Since the window of not present is so small,
issues could lurk for some time. I guess that should show up with XPFO
too though.

> > 2. At least on x86, the obvious algorithm to do the free operation
> > with a single flush requires it.  Someone should probably confirm
> > that
> > arm’s TLB works the same way, i.e. that no flush is needed when
> > changing from not-present (or whatever ARM calls it) to RW.
> > 
> 
> Good point. ARM is similar in this regard, although we'll probably
> clear the access flag rather than unmap the page entirely (which is
> treated the same way in terms of required TLB management)
How about set_alias_nv(not valid)/set_alias_default for the name? It
can cover the general behavior of not cacheable in the TLB.

Also, FYI for anyone that is following this - Nadav and I have
discussed merging this with the text poke patchset because of the
overlap. With the US holidays, I may not get this done and tested until
first week of January. I'll go back and make the efficient direct map
permissions part arch generic now too.

> > 3. Anyone playing with XPFO wants this facility anyway.  In fact,
> > with
> > this change, Rick’s series will more or less implement XPFO for
> > vmalloc memory :)
> > 
> > Does that seem reasonable to you?
> 
> Absolutely.

Ard Biesheuvel Dec. 22, 2018, 11:12 a.m. UTC | #12

On Fri, 21 Dec 2018 at 20:57, Edgecombe, Rick P
<[e-mail]> wrote:
>
> On Fri, 2018-12-21 at 18:25 +0100, Ard Biesheuvel wrote:
> > On Fri, 21 Dec 2018 at 18:12, Andy Lutomirski <[e-mail]>
> > wrote:
> > > > On Dec 21, 2018, at 9:39 AM, Ard Biesheuvel <
> > > > [e-mail]> wrote:
> > > >
> > > > > On Wed, 12 Dec 2018 at 03:20, Andy Lutomirski <[e-mail]>
> > > > > wrote:
> > > > >
> > > > > On Tue, Dec 11, 2018 at 4:12 PM Rick Edgecombe
> > > > > <[e-mail]> wrote:
> > > > > > This adds two new flags VM_IMMEDIATE_UNMAP and
> > > > > > VM_HAS_SPECIAL_PERMS, for
> > > > > > enabling vfree operations to immediately clear executable TLB
> > > > > > entries to freed
> > > > > > pages, and handle freeing memory with special permissions.
> > > > > >
> > > > > > In order to support vfree being called on memory that might
> > > > > > be RO, the vfree
> > > > > > deferred list node is moved to a kmalloc allocated struct,
> > > > > > from where it is
> > > > > > today, reusing the allocation being freed.
> > > > > >
> > > > > > arch_vunmap is a new __weak function that implements the
> > > > > > actual unmapping and
> > > > > > resetting of the direct map permissions. It can be overridden
> > > > > > by more efficient
> > > > > > architecture specific implementations.
> > > > > >
> > > > > > For the default implementation, it uses architecture agnostic
> > > > > > methods which are
> > > > > > equivalent to what most usages do before calling vfree. So
> > > > > > now it is just
> > > > > > centralized here.
> > > > > >
> > > > > > This implementation derives from two sketches from Dave
> > > > > > Hansen and Andy
> > > > > > Lutomirski.
> > > > > >
> > > > > > Suggested-by: Dave Hansen <[e-mail]>
> > > > > > Suggested-by: Andy Lutomirski <[e-mail]>
> > > > > > Suggested-by: Will Deacon <[e-mail]>
> > > > > > Signed-off-by: Rick Edgecombe <[e-mail]>
> > > > > > ---
> > > > > > include/linux/vmalloc.h |  2 ++
> > > > > > mm/vmalloc.c            | 73
> > > > > > +++++++++++++++++++++++++++++++++++++----
> > > > > > 2 files changed, 69 insertions(+), 6 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/vmalloc.h
> > > > > > b/include/linux/vmalloc.h
> > > > > > index 398e9c95cd61..872bcde17aca 100644
> > > > > > --- a/include/linux/vmalloc.h
> > > > > > +++ b/include/linux/vmalloc.h
> > > > > > @@ -21,6 +21,8 @@ struct notifier_block;                /* in
> > > > > > notifier.h */
> > > > > > #define VM_UNINITIALIZED       0x00000020      /* vm_struct
> > > > > > is not fully initialized */
> > > > > > #define VM_NO_GUARD            0x00000040      /* don't add
> > > > > > guard page */
> > > > > > #define VM_KASAN               0x00000080      /* has
> > > > > > allocated kasan shadow memory */
> > > > > > +#define VM_IMMEDIATE_UNMAP     0x00000200      /* flush
> > > > > > before releasing pages */
> > > > > > +#define VM_HAS_SPECIAL_PERMS   0x00000400      /* may be
> > > > > > freed with special perms */
> > > > > > /* bits [20..32] reserved for arch specific ioremap internals
> > > > > > */
> > > > > >
> > > > > > /*
> > > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > > index 97d4b25d0373..02b284d2245a 100644
> > > > > > --- a/mm/vmalloc.c
> > > > > > +++ b/mm/vmalloc.c
> > > > > > @@ -18,6 +18,7 @@
> > > > > > #include <linux/interrupt.h>
> > > > > > #include <linux/proc_fs.h>
> > > > > > #include <linux/seq_file.h>
> > > > > > +#include <linux/set_memory.h>
> > > > > > #include <linux/debugobjects.h>
> > > > > > #include <linux/kallsyms.h>
> > > > > > #include <linux/list.h>
> > > > > > @@ -38,6 +39,11 @@
> > > > > >
> > > > > > #include "internal.h"
> > > > > >
> > > > > > +struct vfree_work {
> > > > > > +       struct llist_node node;
> > > > > > +       void *addr;
> > > > > > +};
> > > > > > +
> > > > > > struct vfree_deferred {
> > > > > >        struct llist_head list;
> > > > > >        struct work_struct wq;
> > > > > > @@ -50,9 +56,13 @@ static void free_work(struct work_struct
> > > > > > *w)
> > > > > > {
> > > > > >        struct vfree_deferred *p = container_of(w, struct
> > > > > > vfree_deferred, wq);
> > > > > >        struct llist_node *t, *llnode;
> > > > > > +       struct vfree_work *cur;
> > > > > >
> > > > > > -       llist_for_each_safe(llnode, t, llist_del_all(&p-
> > > > > > >list))
> > > > > > -               __vunmap((void *)llnode, 1);
> > > > > > +       llist_for_each_safe(llnode, t, llist_del_all(&p-
> > > > > > >list)) {
> > > > > > +               cur = container_of(llnode, struct vfree_work,
> > > > > > node);
> > > > > > +               __vunmap(cur->addr, 1);
> > > > > > +               kfree(cur);
> > > > > > +       }
> > > > > > }
> > > > > >
> > > > > > /*** Page table manipulation functions ***/
> > > > > > @@ -1494,6 +1504,48 @@ struct vm_struct *remove_vm_area(const
> > > > > > void *addr)
> > > > > >        return NULL;
> > > > > > }
> > > > > >
> > > > > > +/*
> > > > > > + * This function handles unmapping and resetting the direct
> > > > > > map as efficiently
> > > > > > + * as it can with cross arch functions. The three categories
> > > > > > of architectures
> > > > > > + * are:
> > > > > > + *   1. Architectures with no set_memory implementations and
> > > > > > no direct map
> > > > > > + *      permissions.
> > > > > > + *   2. Architectures with set_memory implementations but no
> > > > > > direct map
> > > > > > + *      permissions
> > > > > > + *   3. Architectures with set_memory implementations and
> > > > > > direct map permissions
> > > > > > + */
> > > > > > +void __weak arch_vunmap(struct vm_struct *area, int
> > > > > > deallocate_pages)
> > > > >
> > > > > My general preference is to avoid __weak functions -- they
> > > > > don't
> > > > > optimize well.  Instead, I prefer either:
> > > > >
> > > > > #ifndef arch_vunmap
> > > > > void arch_vunmap(...);
> > > > > #endif
> > > > >
> > > > > or
> > > > >
> > > > > #ifdef CONFIG_HAVE_ARCH_VUNMAP
> > > > > ...
> > > > > #endif
> > > > >
> > > > >
> > > > > > +{
> > > > > > +       unsigned long addr = (unsigned long)area->addr;
> > > > > > +       int immediate = area->flags & VM_IMMEDIATE_UNMAP;
> > > > > > +       int special = area->flags & VM_HAS_SPECIAL_PERMS;
> > > > > > +
> > > > > > +       /*
> > > > > > +        * In case of 2 and 3, use this general way of
> > > > > > resetting the permissions
> > > > > > +        * on the directmap. Do NX before RW, in case of X,
> > > > > > so there is no W^X
> > > > > > +        * violation window.
> > > > > > +        *
> > > > > > +        * For case 1 these will be noops.
> > > > > > +        */
> > > > > > +       if (immediate)
> > > > > > +               set_memory_nx(addr, area->nr_pages);
> > > > > > +       if (deallocate_pages && special)
> > > > > > +               set_memory_rw(addr, area->nr_pages);
> > > > >
> > > > > Can you elaborate on the intent here?  VM_IMMEDIATE_UNMAP means
> > > > > "I
> > > > > want that alias gone before any deallocation happens".
> > > > > VM_HAS_SPECIAL_PERMS means "I mucked with the direct map -- fix
> > > > > it for
> > > > > me, please".  deallocate means "this was vfree -- please free
> > > > > the
> > > > > pages".  I'm not convinced that all the various combinations
> > > > > make
> > > > > sense.  Do we really need both flags?
> > > > >
> > > > > (VM_IMMEDIATE_UNMAP is a bit of a lie, since, if
> > > > > in_interrupt(), it's
> > > > > not immediate.)
> > > > >
> > > > > If we do keep both flags, maybe some restructuring would make
> > > > > sense,
> > > > > like this, perhaps.  Sorry about horrible whitespace damage.
> > > > >
> > > > > if (special) {
> > > > >  /* VM_HAS_SPECIAL_PERMS makes little sense without
> > > > > deallocate_pages. */
> > > > >  WARN_ON_ONCE(!deallocate_pages);
> > > > >
> > > > >  if (immediate) {
> > > > >    /* It's possible that the vmap alias is X and we're about to
> > > > > make
> > > > > the direct map RW.  To avoid a window where executable memory
> > > > > is
> > > > > writable, first mark the vmap alias NX.  This is silly, since
> > > > > we're
> > > > > about to *unmap* it, but this is the best we can do if all we
> > > > > have to
> > > > > work with is the set_memory_abc() APIs.  Architectures should
> > > > > override
> > > > > this whole function to get better behavior. */
> > > >
> > > > So can't we fix this first? Assuming that architectures that
> > > > bother to
> > > > implement them will not have executable mappings in the linear
> > > > region,
> > > > all we'd need is set_linear_range_ro/rw() routines that default
> > > > to
> > > > doing nothing, and encapsulate the existing code for x86 and
> > > > arm64.
> > > > That way, we can handle do things in the proper order, i.e.,
> > > > release
> > > > the vmalloc mapping (without caring about the permissions),
> > > > restore
> > > > the linear alias attributes, and finally release the pages.
> > >
> > > Seems reasonable, except that I think it should be
> > > set_linear_range_not_present() and set_linear_range_rw(), for three
> > > reasons:
> > >
> > > 1. It’s not at all clear to me that we need to keep the linear
> > > mapping
> > > around for modules.
> > >
> >
> > I'm pretty sure hibernate on arm64 will have to be fixed, since it
> > expects to be able to read all valid pages via the linear map. But we
> > can fix that.
> Hmm, now I wonder what else might be trying to access the entire direct
> map for some reason. Since the window of not present is so small,
> issues could lurk for some time. I guess that should show up with XPFO
> too though.
>

I don't think there is usually a need to scan the entire address space
like that, unless you are trying to preserve the contents and write
them to disk, like in the hibernate case.

However, IIUC, hibernate on arm64 can already deal with
debug_pagealloc, which relies on clearing the access flag as well, so
if we stick with that we should be ok, I guess.

> > > 2. At least on x86, the obvious algorithm to do the free operation
> > > with a single flush requires it.  Someone should probably confirm
> > > that
> > > arm’s TLB works the same way, i.e. that no flush is needed when
> > > changing from not-present (or whatever ARM calls it) to RW.
> > >
> >
> > Good point. ARM is similar in this regard, although we'll probably
> > clear the access flag rather than unmap the page entirely (which is
> > treated the same way in terms of required TLB management)
> How about set_alias_nv(not valid)/set_alias_default for the name? It
> can cover the general behavior of not cacheable in the TLB.
>

Works for me

> Also, FYI for anyone that is following this - Nadav and I have
> discussed merging this with the text poke patchset because of the
> overlap. With the US holidays, I may not get this done and tested until
> first week of January. I'll go back and make the efficient direct map
> permissions part arch generic now too.
>

Excellent!

> > > 3. Anyone playing with XPFO wants this facility anyway.  In fact,
> > > with
> > > this change, Rick’s series will more or less implement XPFO for
> > > vmalloc memory :)
> > >
> > > Does that seem reasonable to you?
> >
> > Absolutely.

Patch

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 398e9c95cd61..872bcde17aca 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -21,6 +21,8 @@  struct notifier_block;		/* in notifier.h */
 #define VM_UNINITIALIZED	0x00000020	/* vm_struct is not fully initialized */
 #define VM_NO_GUARD		0x00000040      /* don't add guard page */
 #define VM_KASAN		0x00000080      /* has allocated kasan shadow memory */
+#define VM_IMMEDIATE_UNMAP	0x00000200	/* flush before releasing pages */
+#define VM_HAS_SPECIAL_PERMS	0x00000400	/* may be freed with special perms */
 /* bits [20..32] reserved for arch specific ioremap internals */
 
 /*
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 97d4b25d0373..02b284d2245a 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -18,6 +18,7 @@ 
 #include <linux/interrupt.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
+#include <linux/set_memory.h>
 #include <linux/debugobjects.h>
 #include <linux/kallsyms.h>
 #include <linux/list.h>
@@ -38,6 +39,11 @@ 
 
 #include "internal.h"
 
+struct vfree_work {
+	struct llist_node node;
+	void *addr;
+};
+
 struct vfree_deferred {
 	struct llist_head list;
 	struct work_struct wq;
@@ -50,9 +56,13 @@  static void free_work(struct work_struct *w)
 {
 	struct vfree_deferred *p = container_of(w, struct vfree_deferred, wq);
 	struct llist_node *t, *llnode;
+	struct vfree_work *cur;
 
-	llist_for_each_safe(llnode, t, llist_del_all(&p->list))
-		__vunmap((void *)llnode, 1);
+	llist_for_each_safe(llnode, t, llist_del_all(&p->list)) {
+		cur = container_of(llnode, struct vfree_work, node);
+		__vunmap(cur->addr, 1);
+		kfree(cur);
+	}
 }
 
 /*** Page table manipulation functions ***/
@@ -1494,6 +1504,48 @@  struct vm_struct *remove_vm_area(const void *addr)
 	return NULL;
 }
 
+/*
+ * This function handles unmapping and resetting the direct map as efficiently
+ * as it can with cross arch functions. The three categories of architectures
+ * are:
+ *   1. Architectures with no set_memory implementations and no direct map
+ *      permissions.
+ *   2. Architectures with set_memory implementations but no direct map
+ *      permissions
+ *   3. Architectures with set_memory implementations and direct map permissions
+ */
+void __weak arch_vunmap(struct vm_struct *area, int deallocate_pages)
+{
+	unsigned long addr = (unsigned long)area->addr;
+	int immediate = area->flags & VM_IMMEDIATE_UNMAP;
+	int special = area->flags & VM_HAS_SPECIAL_PERMS;
+
+	/*
+	 * In case of 2 and 3, use this general way of resetting the permissions
+	 * on the directmap. Do NX before RW, in case of X, so there is no W^X
+	 * violation window.
+	 *
+	 * For case 1 these will be noops.
+	 */
+	if (immediate)
+		set_memory_nx(addr, area->nr_pages);
+	if (deallocate_pages && special)
+		set_memory_rw(addr, area->nr_pages);
+
+	/* Always actually remove the area */
+	remove_vm_area(area->addr);
+
+	/*
+	 * Need to flush the TLB before freeing pages in the case of this flag.
+	 * As long as that's happening, unmap aliases.
+	 *
+	 * For 2 and 3, this will not be needed because of the set_memory_nx
+	 * above, because the stale TLBs will be NX.
+	 */
+	if (immediate && !IS_ENABLED(ARCH_HAS_SET_MEMORY))
+		vm_unmap_aliases();
+}
+
 static void __vunmap(const void *addr, int deallocate_pages)
 {
 	struct vm_struct *area;
@@ -1515,7 +1567,8 @@  static void __vunmap(const void *addr, int deallocate_pages)
 	debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
 	debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
 
-	remove_vm_area(addr);
+	arch_vunmap(area, deallocate_pages);
+
 	if (deallocate_pages) {
 		int i;
 
@@ -1542,8 +1595,15 @@  static inline void __vfree_deferred(const void *addr)
 	 * nother cpu's list.  schedule_work() should be fine with this too.
 	 */
 	struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
+	struct vfree_work *w = kmalloc(sizeof(struct vfree_work), GFP_ATOMIC);
+
+	/* If no memory for the deferred list node, give up */
+	if (!w)
+		return;
 
-	if (llist_add((struct llist_node *)addr, &p->list))
+	w->addr = (void *)addr;
+
+	if (llist_add(&w->node, &p->list))
 		schedule_work(&p->wq);
 }
 
@@ -1925,8 +1985,9 @@  EXPORT_SYMBOL(vzalloc_node);
 
 void *vmalloc_exec(unsigned long size)
 {
-	return __vmalloc_node(size, 1, GFP_KERNEL, PAGE_KERNEL_EXEC,
-			      NUMA_NO_NODE, __builtin_return_address(0));
+	return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
+			GFP_KERNEL, PAGE_KERNEL_EXEC, VM_IMMEDIATE_UNMAP,
+			NUMA_NO_NODE, __builtin_return_address(0));
 }
 
 #if defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA32)