[v2,06/20] x86/alternative: use temporary mm for text poking
diff mbox series

Message ID 20190129003422.9328-7-rick.p.edgecombe@intel.com
State New
Headers show
Series
  • Merge text_poke fixes and executable lockdowns
Related show

Commit Message

Edgecombe, Rick P Jan. 29, 2019, 12:34 a.m. UTC
From: Nadav Amit <namit@vmware.com>

text_poke() can potentially compromise the security as it sets temporary
PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
from other cores accidentally or maliciously, if an attacker gains the
ability to write onto kernel memory.

Moreover, since remote TLBs are not flushed after the temporary PTEs are
removed, the time-window in which the code is writable is not limited if
the fixmap PTEs - maliciously or accidentally - are cached in the TLB.
To address these potential security hazards, we use a temporary mm for
patching the code.

Finally, text_poke() is also not conservative enough when mapping pages,
as it always tries to map 2 pages, even when a single one is sufficient.
So try to be more conservative, and do not map more than needed.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
 arch/x86/include/asm/fixmap.h |   2 -
 arch/x86/kernel/alternative.c | 106 +++++++++++++++++++++++++++-------
 arch/x86/xen/mmu_pv.c         |   2 -
 3 files changed, 84 insertions(+), 26 deletions(-)

Comments

Borislav Petkov Feb. 5, 2019, 9:58 a.m. UTC | #1
On Mon, Jan 28, 2019 at 04:34:08PM -0800, Rick Edgecombe wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> text_poke() can potentially compromise the security as it sets temporary

s/the //

> PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
> from other cores accidentally or maliciously, if an attacker gains the
> ability to write onto kernel memory.

Eww, sneaky. That would be a really nasty attack.

> Moreover, since remote TLBs are not flushed after the temporary PTEs are
> removed, the time-window in which the code is writable is not limited if
> the fixmap PTEs - maliciously or accidentally - are cached in the TLB.
> To address these potential security hazards, we use a temporary mm for
> patching the code.
> 
> Finally, text_poke() is also not conservative enough when mapping pages,
> as it always tries to map 2 pages, even when a single one is sufficient.
> So try to be more conservative, and do not map more than needed.
> 
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---
>  arch/x86/include/asm/fixmap.h |   2 -
>  arch/x86/kernel/alternative.c | 106 +++++++++++++++++++++++++++-------
>  arch/x86/xen/mmu_pv.c         |   2 -
>  3 files changed, 84 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index 50ba74a34a37..9da8cccdf3fb 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -103,8 +103,6 @@ enum fixed_addresses {
>  #ifdef CONFIG_PARAVIRT
>  	FIX_PARAVIRT_BOOTMAP,
>  #endif
> -	FIX_TEXT_POKE1,	/* reserve 2 pages for text_poke() */
> -	FIX_TEXT_POKE0, /* first page is last, because allocation is backward */

Two fixmap slots less - good riddance. :)

>  #ifdef	CONFIG_X86_INTEL_MID
>  	FIX_LNW_VRTC,
>  #endif
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index ae05fbb50171..76d482a2b716 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -11,6 +11,7 @@
>  #include <linux/stop_machine.h>
>  #include <linux/slab.h>
>  #include <linux/kdebug.h>
> +#include <linux/mmu_context.h>
>  #include <asm/text-patching.h>
>  #include <asm/alternative.h>
>  #include <asm/sections.h>
> @@ -683,41 +684,102 @@ __ro_after_init unsigned long poking_addr;
>  
>  static void *__text_poke(void *addr, const void *opcode, size_t len)
>  {
> +	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
> +	temporary_mm_state_t prev;
> +	struct page *pages[2] = {NULL};
>  	unsigned long flags;
> -	char *vaddr;
> -	struct page *pages[2];
> -	int i;
> +	pte_t pte, *ptep;
> +	spinlock_t *ptl;
> +	pgprot_t prot;
>  
>  	/*
> -	 * While boot memory allocator is runnig we cannot use struct
> -	 * pages as they are not yet initialized.
> +	 * While boot memory allocator is running we cannot use struct pages as
> +	 * they are not yet initialized.
>  	 */
>  	BUG_ON(!after_bootmem);
>  
>  	if (!core_kernel_text((unsigned long)addr)) {
>  		pages[0] = vmalloc_to_page(addr);
> -		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
> +		if (cross_page_boundary)
> +			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
>  	} else {
>  		pages[0] = virt_to_page(addr);
>  		WARN_ON(!PageReserved(pages[0]));
> -		pages[1] = virt_to_page(addr + PAGE_SIZE);
> +		if (cross_page_boundary)
> +			pages[1] = virt_to_page(addr + PAGE_SIZE);
>  	}
> -	BUG_ON(!pages[0]);
> +	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));

checkpatch fires a lot for this patchset and I think we should tone down
the BUG_ON() use.

WARNING: Avoid crashing the kernel - try using WARN_ON & recovery code rather than BUG() or BUG_ON()
#116: FILE: arch/x86/kernel/alternative.c:711:
+       BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));

While the below BUG_ON makes sense, this here could be a WARN_ON or so.

Which begs the next question: AFAICT, nothing looks at text_poke*()'s
retval. So why are we even bothering returning something?

> +
>  	local_irq_save(flags);
> -	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
> -	if (pages[1])
> -		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
> -	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
> -	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
> -	clear_fixmap(FIX_TEXT_POKE0);
> -	if (pages[1])
> -		clear_fixmap(FIX_TEXT_POKE1);
> -	local_flush_tlb();
> -	sync_core();
> -	/* Could also do a CLFLUSH here to speed up CPU recovery; but
> -	   that causes hangs on some VIA CPUs. */
> -	for (i = 0; i < len; i++)
> -		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
> +
> +	/*
> +	 * The lock is not really needed, but this allows to avoid open-coding.
> +	 */
> +	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
> +
> +	/*
> +	 * This must not fail; preallocated in poking_init().
> +	 */
> +	VM_BUG_ON(!ptep);
> +
> +	/*
> +	 * flush_tlb_mm_range() would be called when the poking_mm is not
> +	 * loaded. When PCID is in use, the flush would be deferred to the time
> +	 * the poking_mm is loaded again. Set the PTE as non-global to prevent
> +	 * it from being used when we are done.
> +	 */
> +	prot = __pgprot(pgprot_val(PAGE_KERNEL) & ~_PAGE_GLOBAL);

So

				_KERNPG_TABLE | _PAGE_NX

as this is pagetable page, AFAICT.
Peter Zijlstra Feb. 5, 2019, 11:31 a.m. UTC | #2
On Tue, Feb 05, 2019 at 10:58:53AM +0100, Borislav Petkov wrote:
> > @@ -683,41 +684,102 @@ __ro_after_init unsigned long poking_addr;
> >  
> >  static void *__text_poke(void *addr, const void *opcode, size_t len)
> >  {
> > +	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
> > +	temporary_mm_state_t prev;
> > +	struct page *pages[2] = {NULL};
> >  	unsigned long flags;
> > -	char *vaddr;
> > -	struct page *pages[2];
> > -	int i;
> > +	pte_t pte, *ptep;
> > +	spinlock_t *ptl;
> > +	pgprot_t prot;
> >  
> >  	/*
> > -	 * While boot memory allocator is runnig we cannot use struct
> > -	 * pages as they are not yet initialized.
> > +	 * While boot memory allocator is running we cannot use struct pages as
> > +	 * they are not yet initialized.
> >  	 */
> >  	BUG_ON(!after_bootmem);
> >  
> >  	if (!core_kernel_text((unsigned long)addr)) {
> >  		pages[0] = vmalloc_to_page(addr);
> > -		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
> > +		if (cross_page_boundary)
> > +			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
> >  	} else {
> >  		pages[0] = virt_to_page(addr);
> >  		WARN_ON(!PageReserved(pages[0]));
> > -		pages[1] = virt_to_page(addr + PAGE_SIZE);
> > +		if (cross_page_boundary)
> > +			pages[1] = virt_to_page(addr + PAGE_SIZE);
> >  	}
> > -	BUG_ON(!pages[0]);
> > +	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));
> 
> checkpatch fires a lot for this patchset and I think we should tone down
> the BUG_ON() use.

I've been pushing for BUG_ON() in this patch set; sod checkpatch.

Maybe not this BUG_ON in particular, but a number of them introduced
here are really situations where we can't do anything sane.

This BUG_ON() in particular is the choice between corrupted text or an
instantly dead machine; what would you do?

In general, text_poke() cannot fail:

 - suppose changing a single jump label requires poking multiple sites
   (not uncommon), we fail halfway through and then have to undo the
   first pokes, but those pokes fail again.

 - this then leaves us no way forward and no way back, we've got
   inconsistent text state -> FAIL.

So even an 'early' fail (like here) doesn't work in the rollback
scenario if you combine them.

So while in general I agree with BUG_ON() being undesirable, I think
liberal sprinking in text_poke() is fine; you really _REALLY_ want this
to work or fail loudly. Text corruption is just painful.
Borislav Petkov Feb. 5, 2019, 12:35 p.m. UTC | #3
On Tue, Feb 05, 2019 at 12:31:46PM +0100, Peter Zijlstra wrote:
> ...
>
> So while in general I agree with BUG_ON() being undesirable, I think
> liberal sprinking in text_poke() is fine; you really _REALLY_ want this
> to work or fail loudly. Text corruption is just painful.

Ok. It would be good to have the gist of this sentiment in a comment
above it so that it is absolutely clear why we're doing it.

And since text_poke() can't fail, then it doesn't need a retval too.
AFAICT, nothing is actually using it.
Peter Zijlstra Feb. 5, 2019, 1:25 p.m. UTC | #4
On Tue, Feb 05, 2019 at 01:35:33PM +0100, Borislav Petkov wrote:
> On Tue, Feb 05, 2019 at 12:31:46PM +0100, Peter Zijlstra wrote:
> > ...
> >
> > So while in general I agree with BUG_ON() being undesirable, I think
> > liberal sprinking in text_poke() is fine; you really _REALLY_ want this
> > to work or fail loudly. Text corruption is just painful.
> 
> Ok. It would be good to have the gist of this sentiment in a comment
> above it so that it is absolutely clear why we're doing it.
> 
> And since text_poke() can't fail, then it doesn't need a retval too.
> AFAICT, nothing is actually using it.

See patch 12, that removes the return value (after fixing the few users
that currently 'rely' on it).
Peter Zijlstra Feb. 5, 2019, 1:29 p.m. UTC | #5
On Tue, Feb 05, 2019 at 12:31:46PM +0100, Peter Zijlstra wrote:
> In general, text_poke() cannot fail:
> 
>  - suppose changing a single jump label requires poking multiple sites
>    (not uncommon), we fail halfway through and then have to undo the
>    first pokes, but those pokes fail again.
> 
>  - this then leaves us no way forward and no way back, we've got
>    inconsistent text state -> FAIL.

Note that this exact fail scenario still exists in the CPU hotplug code.
See kernel/cpu.c:cpuhp_thread_fun():

		/*
		 * If we fail on a rollback, we're up a creek without no
		 * paddle, no way forward, no way back. We loose, thanks for
		 * playing.
		 */
		WARN_ON_ONCE(st->rollback);
Nadav Amit Feb. 5, 2019, 5:54 p.m. UTC | #6
> On Feb 5, 2019, at 4:35 AM, Borislav Petkov <bp@alien8.de> wrote:
> 
> On Tue, Feb 05, 2019 at 12:31:46PM +0100, Peter Zijlstra wrote:
>> ...
>> 
>> So while in general I agree with BUG_ON() being undesirable, I think
>> liberal sprinking in text_poke() is fine; you really _REALLY_ want this
>> to work or fail loudly. Text corruption is just painful.
> 
> Ok. It would be good to have the gist of this sentiment in a comment
> above it so that it is absolutely clear why we're doing it.

I added a short comment for v3 above each BUG_ON().

> And since text_poke() can't fail, then it doesn't need a retval too.
> AFAICT, nothing is actually using it.

As Peter said, this is addressed in a separate patch (one patch per logical
change).

Patch
diff mbox series

diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
index 50ba74a34a37..9da8cccdf3fb 100644
--- a/arch/x86/include/asm/fixmap.h
+++ b/arch/x86/include/asm/fixmap.h
@@ -103,8 +103,6 @@  enum fixed_addresses {
 #ifdef CONFIG_PARAVIRT
 	FIX_PARAVIRT_BOOTMAP,
 #endif
-	FIX_TEXT_POKE1,	/* reserve 2 pages for text_poke() */
-	FIX_TEXT_POKE0, /* first page is last, because allocation is backward */
 #ifdef	CONFIG_X86_INTEL_MID
 	FIX_LNW_VRTC,
 #endif
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index ae05fbb50171..76d482a2b716 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -11,6 +11,7 @@ 
 #include <linux/stop_machine.h>
 #include <linux/slab.h>
 #include <linux/kdebug.h>
+#include <linux/mmu_context.h>
 #include <asm/text-patching.h>
 #include <asm/alternative.h>
 #include <asm/sections.h>
@@ -683,41 +684,102 @@  __ro_after_init unsigned long poking_addr;
 
 static void *__text_poke(void *addr, const void *opcode, size_t len)
 {
+	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
+	temporary_mm_state_t prev;
+	struct page *pages[2] = {NULL};
 	unsigned long flags;
-	char *vaddr;
-	struct page *pages[2];
-	int i;
+	pte_t pte, *ptep;
+	spinlock_t *ptl;
+	pgprot_t prot;
 
 	/*
-	 * While boot memory allocator is runnig we cannot use struct
-	 * pages as they are not yet initialized.
+	 * While boot memory allocator is running we cannot use struct pages as
+	 * they are not yet initialized.
 	 */
 	BUG_ON(!after_bootmem);
 
 	if (!core_kernel_text((unsigned long)addr)) {
 		pages[0] = vmalloc_to_page(addr);
-		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
 	} else {
 		pages[0] = virt_to_page(addr);
 		WARN_ON(!PageReserved(pages[0]));
-		pages[1] = virt_to_page(addr + PAGE_SIZE);
+		if (cross_page_boundary)
+			pages[1] = virt_to_page(addr + PAGE_SIZE);
 	}
-	BUG_ON(!pages[0]);
+	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));
+
 	local_irq_save(flags);
-	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
-	if (pages[1])
-		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
-	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
-	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
-	clear_fixmap(FIX_TEXT_POKE0);
-	if (pages[1])
-		clear_fixmap(FIX_TEXT_POKE1);
-	local_flush_tlb();
-	sync_core();
-	/* Could also do a CLFLUSH here to speed up CPU recovery; but
-	   that causes hangs on some VIA CPUs. */
-	for (i = 0; i < len; i++)
-		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
+
+	/*
+	 * The lock is not really needed, but this allows to avoid open-coding.
+	 */
+	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
+
+	/*
+	 * This must not fail; preallocated in poking_init().
+	 */
+	VM_BUG_ON(!ptep);
+
+	/*
+	 * flush_tlb_mm_range() would be called when the poking_mm is not
+	 * loaded. When PCID is in use, the flush would be deferred to the time
+	 * the poking_mm is loaded again. Set the PTE as non-global to prevent
+	 * it from being used when we are done.
+	 */
+	prot = __pgprot(pgprot_val(PAGE_KERNEL) & ~_PAGE_GLOBAL);
+
+	pte = mk_pte(pages[0], prot);
+	set_pte_at(poking_mm, poking_addr, ptep, pte);
+
+	if (cross_page_boundary) {
+		pte = mk_pte(pages[1], prot);
+		set_pte_at(poking_mm, poking_addr + PAGE_SIZE, ptep + 1, pte);
+	}
+
+	/*
+	 * Loading the temporary mm behaves as a compiler barrier, which
+	 * guarantees that the PTE will be set at the time memcpy() is done.
+	 */
+	prev = use_temporary_mm(poking_mm);
+
+	kasan_disable_current();
+	memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
+	kasan_enable_current();
+
+	/*
+	 * Ensure that the PTE is only cleared after the instructions of memcpy
+	 * were issued by using a compiler barrier.
+	 */
+	barrier();
+
+	pte_clear(poking_mm, poking_addr, ptep);
+	if (cross_page_boundary)
+		pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
+
+	/*
+	 * Loading the previous page-table hierarchy requires a serializing
+	 * instruction that already allows the core to see the updated version.
+	 * Xen-PV is assumed to serialize execution in a similar manner.
+	 */
+	unuse_temporary_mm(prev);
+
+	/*
+	 * Flushing the TLB might involve IPIs, which would require enabled
+	 * IRQs, but not if the mm is not used, as it is in this point.
+	 */
+	flush_tlb_mm_range(poking_mm, poking_addr, poking_addr +
+			   (cross_page_boundary ? 2 : 1) * PAGE_SIZE,
+			   PAGE_SHIFT, false);
+
+	pte_unmap_unlock(ptep, ptl);
+	/*
+	 * If the text doesn't match what we just wrote; something is
+	 * fundamentally screwy, there's nothing we can really do about that.
+	 */
+	BUG_ON(memcmp(addr, opcode, len));
+
 	local_irq_restore(flags);
 	return addr;
 }
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 0f4fe206dcc2..82b181fcefe5 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -2319,8 +2319,6 @@  static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
 #elif defined(CONFIG_X86_VSYSCALL_EMULATION)
 	case VSYSCALL_PAGE:
 #endif
-	case FIX_TEXT_POKE0:
-	case FIX_TEXT_POKE1:
 		/* All local page mappings */
 		pte = pfn_pte(phys, prot);
 		break;