diff mbox series

[v5,08/15] mm: Add unsafe_follow_pfn

Message ID 20201030100815.2269-9-daniel.vetter@ffwll.ch
State New, archived
Headers show
Series follow_pfn and other iomap races | expand

Commit Message

Daniel Vetter Oct. 30, 2020, 10:08 a.m. UTC
Way back it was a reasonable assumptions that iomem mappings never
change the pfn range they point at. But this has changed:

- gpu drivers dynamically manage their memory nowadays, invalidating
ptes with unmap_mapping_range when buffers get moved

- contiguous dma allocations have moved from dedicated carvetouts to
cma regions. This means if we miss the unmap the pfn might contain
pagecache or anon memory (well anything allocated with GFP_MOVEABLE)

- even /dev/mem now invalidates mappings when the kernel requests that
iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87
("/dev/mem: Revoke mappings when a driver claims the region")

Accessing pfns obtained from ptes without holding all the locks is
therefore no longer a good idea.

Unfortunately there's some users where this is not fixable (like v4l
userptr of iomem mappings) or involves a pile of work (vfio type1
iommu). For now annotate these as unsafe and splat appropriately.

This patch adds an unsafe_follow_pfn, which later patches will then
roll out to all appropriate places.

Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
that by drivers/modules is together with an mmu_notifier, and that's
all _GPL stuff.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: linux-mm@kvack.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Cc: linux-media@vger.kernel.org
Cc: kvm@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
--
v5: Suggestions from Christoph
- reindent for less weirdness
- use IS_ENABLED instead of #ifdef
- same checks for nommu, for consistency
- EXPORT_SYMBOL_GPL for follow_pfn.
- kerneldoc was already updated in previous versions to explain when
  follow_pfn can be used safely
---
 include/linux/mm.h |  2 ++
 mm/memory.c        | 34 ++++++++++++++++++++++++++++++++--
 mm/nommu.c         | 27 ++++++++++++++++++++++++++-
 security/Kconfig   | 13 +++++++++++++
 4 files changed, 73 insertions(+), 3 deletions(-)

Comments

Daniel Vetter Nov. 2, 2020, 12:56 p.m. UTC | #1
On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote:
> > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> > that by drivers/modules is together with an mmu_notifier, and that's
> > all _GPL stuff.
>
> I also think it also needs to be renamed to explicitly break any existing
> users out of tree or int the submission queue.

Ok I looked at the mmu notifier locking again and noticed that
mm->subscriptions has its own spinlock. Since there usually shouldn't
be a huge pile of these I think it's feasible to check for the mmu
notifier in follow_pfn. And that would stuff this gap for good. I'll
throw that on top as a final patch and see what people think.
-Daniel
Jason Gunthorpe Nov. 2, 2020, 1:01 p.m. UTC | #2
On Mon, Nov 02, 2020 at 01:56:10PM +0100, Daniel Vetter wrote:
> On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote:
> > > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> > > that by drivers/modules is together with an mmu_notifier, and that's
> > > all _GPL stuff.
> >
> > I also think it also needs to be renamed to explicitly break any existing
> > users out of tree or int the submission queue.
> 
> Ok I looked at the mmu notifier locking again and noticed that
> mm->subscriptions has its own spinlock. Since there usually shouldn't
> be a huge pile of these I think it's feasible to check for the mmu
> notifier in follow_pfn. And that would stuff this gap for good. I'll
> throw that on top as a final patch and see what people think.

Probably the simplest is to just check mm_has_notifiers() when in
lockdep or something very simple like that

Jason
Daniel Vetter Nov. 2, 2020, 1:23 p.m. UTC | #3
On Mon, Nov 2, 2020 at 2:01 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Mon, Nov 02, 2020 at 01:56:10PM +0100, Daniel Vetter wrote:
> > On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch@infradead.org> wrote:
> > >
> > > On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote:
> > > > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> > > > that by drivers/modules is together with an mmu_notifier, and that's
> > > > all _GPL stuff.
> > >
> > > I also think it also needs to be renamed to explicitly break any existing
> > > users out of tree or int the submission queue.
> >
> > Ok I looked at the mmu notifier locking again and noticed that
> > mm->subscriptions has its own spinlock. Since there usually shouldn't
> > be a huge pile of these I think it's feasible to check for the mmu
> > notifier in follow_pfn. And that would stuff this gap for good. I'll
> > throw that on top as a final patch and see what people think.
>
> Probably the simplest is to just check mm_has_notifiers() when in
> lockdep or something very simple like that

lockdep feels wrong, was locking more at CONFIG_DEBUG_VM. And since
generally you only have 1 mmu notifier (especially for kvm) I think we
can also pay the 2nd cacheline miss and actually check the right mmu
notifier is registered.
-Daniel
Jason Gunthorpe Nov. 2, 2020, 3:52 p.m. UTC | #4
On Mon, Nov 02, 2020 at 02:23:58PM +0100, Daniel Vetter wrote:
> On Mon, Nov 2, 2020 at 2:01 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Mon, Nov 02, 2020 at 01:56:10PM +0100, Daniel Vetter wrote:
> > > On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch@infradead.org> wrote:
> > > >
> > > > On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote:
> > > > > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> > > > > that by drivers/modules is together with an mmu_notifier, and that's
> > > > > all _GPL stuff.
> > > >
> > > > I also think it also needs to be renamed to explicitly break any existing
> > > > users out of tree or int the submission queue.
> > >
> > > Ok I looked at the mmu notifier locking again and noticed that
> > > mm->subscriptions has its own spinlock. Since there usually shouldn't
> > > be a huge pile of these I think it's feasible to check for the mmu
> > > notifier in follow_pfn. And that would stuff this gap for good. I'll
> > > throw that on top as a final patch and see what people think.
> >
> > Probably the simplest is to just check mm_has_notifiers() when in
> > lockdep or something very simple like that
> 
> lockdep feels wrong, was locking more at CONFIG_DEBUG_VM. And since
> generally you only have 1 mmu notifier (especially for kvm) I think we
> can also pay the 2nd cacheline miss and actually check the right mmu
> notifier is registered.

Need to hold the lock to check that and there are two ways to register
notifiers these days, so it feels to expensive to me.

CH's 'export symbol only for kvm' really does seem the most robust way
to handle this though.

Jason
Daniel Vetter Nov. 2, 2020, 4:42 p.m. UTC | #5
On Mon, Nov 2, 2020 at 4:52 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Mon, Nov 02, 2020 at 02:23:58PM +0100, Daniel Vetter wrote:
> > On Mon, Nov 2, 2020 at 2:01 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Mon, Nov 02, 2020 at 01:56:10PM +0100, Daniel Vetter wrote:
> > > > On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch@infradead.org> wrote:
> > > > >
> > > > > On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote:
> > > > > > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> > > > > > that by drivers/modules is together with an mmu_notifier, and that's
> > > > > > all _GPL stuff.
> > > > >
> > > > > I also think it also needs to be renamed to explicitly break any existing
> > > > > users out of tree or int the submission queue.
> > > >
> > > > Ok I looked at the mmu notifier locking again and noticed that
> > > > mm->subscriptions has its own spinlock. Since there usually shouldn't
> > > > be a huge pile of these I think it's feasible to check for the mmu
> > > > notifier in follow_pfn. And that would stuff this gap for good. I'll
> > > > throw that on top as a final patch and see what people think.
> > >
> > > Probably the simplest is to just check mm_has_notifiers() when in
> > > lockdep or something very simple like that
> >
> > lockdep feels wrong, was locking more at CONFIG_DEBUG_VM. And since
> > generally you only have 1 mmu notifier (especially for kvm) I think we
> > can also pay the 2nd cacheline miss and actually check the right mmu
> > notifier is registered.
>
> Need to hold the lock to check that and there are two ways to register
> notifiers these days, so it feels to expensive to me.

Uh I mixed stuff up all along, struct mmu_notifier *subcription that
all the mmu notifier users use has the ->mm pointer we want right
there. That's good enough I think.

Now I'm kinda lost in kvm code trying to wire it through, but it's
looking ok-ish thus far :-)
-Daniel

> CH's 'export symbol only for kvm' really does seem the most robust way
> to handle this though.
>
> Jason
Jason Gunthorpe Nov. 2, 2020, 5:16 p.m. UTC | #6
On Mon, Nov 02, 2020 at 05:42:20PM +0100, Daniel Vetter wrote:
> > Need to hold the lock to check that and there are two ways to register
> > notifiers these days, so it feels to expensive to me.
> 
> Uh I mixed stuff up all along, struct mmu_notifier *subcription that
> all the mmu notifier users use has the ->mm pointer we want right
> there. That's good enough I think.

Yah, if you can pass in one of those instead of the raw mm it would be
fine

Jason
diff mbox series

Patch

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 83d0be101a38..d0fe8bf46a9d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1661,6 +1661,8 @@  int follow_pte_pmd(struct mm_struct *mm, unsigned long address,
 		   pte_t **ptepp, pmd_t **pmdpp, spinlock_t **ptlp);
 int follow_pfn(struct vm_area_struct *vma, unsigned long address,
 	unsigned long *pfn);
+int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,
+		      unsigned long *pfn);
 int follow_phys(struct vm_area_struct *vma, unsigned long address,
 		unsigned int flags, unsigned long *prot, resource_size_t *phys);
 int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
diff --git a/mm/memory.c b/mm/memory.c
index ac32039ce941..0db0c5e233fd 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4795,7 +4795,12 @@  EXPORT_SYMBOL(follow_pte_pmd);
  * @address: user virtual address
  * @pfn: location to store found PFN
  *
- * Only IO mappings and raw PFN mappings are allowed.
+ * Only IO mappings and raw PFN mappings are allowed. Note that callers must
+ * ensure coherency with pte updates by using a &mmu_notifier to follow updates.
+ * If this is not feasible, or the access to the @pfn is only very short term,
+ * use follow_pte_pmd() instead and hold the pagetable lock for the duration of
+ * the access instead. Any caller not following these requirements must use
+ * unsafe_follow_pfn() instead.
  *
  * Return: zero and the pfn at @pfn on success, -ve otherwise.
  */
@@ -4816,7 +4821,32 @@  int follow_pfn(struct vm_area_struct *vma, unsigned long address,
 	pte_unmap_unlock(ptep, ptl);
 	return 0;
 }
-EXPORT_SYMBOL(follow_pfn);
+EXPORT_SYMBOL_GPL(follow_pfn);
+
+/**
+ * unsafe_follow_pfn - look up PFN at a user virtual address
+ * @vma: memory mapping
+ * @address: user virtual address
+ * @pfn: location to store found PFN
+ *
+ * Only IO mappings and raw PFN mappings are allowed.
+ *
+ * Returns zero and the pfn at @pfn on success, -ve otherwise.
+ */
+int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,
+		      unsigned long *pfn)
+{
+	if (IS_ENABLED(CONFIG_STRICT_FOLLOW_PFN)) {
+		pr_info("unsafe follow_pfn usage rejected, see CONFIG_STRICT_FOLLOW_PFN\n");
+		return -EINVAL;
+	}
+
+	WARN_ONCE(1, "unsafe follow_pfn usage\n");
+	add_taint(TAINT_USER, LOCKDEP_STILL_OK);
+
+	return follow_pfn(vma, address, pfn);
+}
+EXPORT_SYMBOL(unsafe_follow_pfn);
 
 #ifdef CONFIG_HAVE_IOREMAP_PROT
 int follow_phys(struct vm_area_struct *vma,
diff --git a/mm/nommu.c b/mm/nommu.c
index 0faf39b32cdb..79fc98a6c94a 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -130,7 +130,32 @@  int follow_pfn(struct vm_area_struct *vma, unsigned long address,
 	*pfn = address >> PAGE_SHIFT;
 	return 0;
 }
-EXPORT_SYMBOL(follow_pfn);
+EXPORT_SYMBOL_GPL(follow_pfn);
+
+/**
+ * unsafe_follow_pfn - look up PFN at a user virtual address
+ * @vma: memory mapping
+ * @address: user virtual address
+ * @pfn: location to store found PFN
+ *
+ * Only IO mappings and raw PFN mappings are allowed.
+ *
+ * Returns zero and the pfn at @pfn on success, -ve otherwise.
+ */
+int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,
+		      unsigned long *pfn)
+{
+	if (IS_ENABLED(CONFIG_STRICT_FOLLOW_PFN)) {
+		pr_info("unsafe follow_pfn usage rejected, see CONFIG_STRICT_FOLLOW_PFN\n");
+		return -EINVAL;
+	}
+
+	WARN_ONCE(1, "unsafe follow_pfn usage\n");
+	add_taint(TAINT_USER, LOCKDEP_STILL_OK);
+
+	return follow_pfn(vma, address, pfn);
+}
+EXPORT_SYMBOL(unsafe_follow_pfn);
 
 LIST_HEAD(vmap_area_list);
 
diff --git a/security/Kconfig b/security/Kconfig
index 7561f6f99f1d..48945402e103 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -230,6 +230,19 @@  config STATIC_USERMODEHELPER_PATH
 	  If you wish for all usermode helper programs to be disabled,
 	  specify an empty string here (i.e. "").
 
+config STRICT_FOLLOW_PFN
+	bool "Disable unsafe use of follow_pfn"
+	depends on MMU
+	help
+	  Some functionality in the kernel follows userspace mappings to iomem
+	  ranges in an unsafe matter. Examples include v4l userptr for zero-copy
+	  buffers sharing.
+
+	  If this option is switched on, such access is rejected. Only enable
+	  this option when you must run userspace which requires this.
+
+	  If in doubt, say Y.
+
 source "security/selinux/Kconfig"
 source "security/smack/Kconfig"
 source "security/tomoyo/Kconfig"