[PATCH] MIPS: math-emu: Write-protect delay slot emulation pages

[PATCH] MIPS: math-emu: Write-protect delay slot emulation pages

[Paul Burton]

Mapping the delay slot emulation page as both writeable & executable
presents a security risk, in that if an exploit can write to & jump into
the page then it can be used as an easy way to execute arbitrary code.

Prevent this by mapping the page read-only for userland, and using
access_process_vm() with the FOLL_FORCE flag to write to it from
mips_dsemul().

This will likely be less efficient due to copy_to_user_page() performing
cache maintenance on a whole page, rather than a single line as in the
previous use of flush_cache_sigtramp(). However this delay slot
emulation code ought not to be running in any performance critical paths
anyway so this isn't really a problem, and we can probably do better in
copy_to_user_page() anyway in future.

A major advantage of this approach is that the fix is small & simple to
backport to stable kernels.

Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")
Cc: stable@vger.kernel.org # v4.8+
---
 arch/mips/kernel/vdso.c     |  4 ++--
 arch/mips/math-emu/dsemul.c | 38 +++++++++++++++++++------------------
 2 files changed, 22 insertions(+), 20 deletions(-)

diff --git a/arch/mips/kernel/vdso.c b/arch/mips/kernel/vdso.c
index 48a9c6b90e07..9df3ebdc7b0f 100644
--- a/arch/mips/kernel/vdso.c
+++ b/arch/mips/kernel/vdso.c
@@ -126,8 +126,8 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 
 	/* Map delay slot emulation page */
 	base = mmap_region(NULL, STACK_TOP, PAGE_SIZE,
-			   VM_READ|VM_WRITE|VM_EXEC|
-			   VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
+			   VM_READ | VM_EXEC |
+			   VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
 			   0, NULL);
 	if (IS_ERR_VALUE(base)) {
 		ret = base;
diff --git a/arch/mips/math-emu/dsemul.c b/arch/mips/math-emu/dsemul.c
index 5450f4d1c920..e2d46cb93ca9 100644
--- a/arch/mips/math-emu/dsemul.c
+++ b/arch/mips/math-emu/dsemul.c
@@ -214,8 +214,9 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
 {
 	int isa16 = get_isa16_mode(regs->cp0_epc);
 	mips_instruction break_math;
-	struct emuframe __user *fr;
-	int err, fr_idx;
+	unsigned long fr_uaddr;
+	struct emuframe fr;
+	int fr_idx, ret;
 
 	/* NOP is easy */
 	if (ir == 0)
@@ -250,27 +251,31 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
 		fr_idx = alloc_emuframe();
 	if (fr_idx == BD_EMUFRAME_NONE)
 		return SIGBUS;
-	fr = &dsemul_page()[fr_idx];
 
 	/* Retrieve the appropriately encoded break instruction */
 	break_math = BREAK_MATH(isa16);
 
 	/* Write the instructions to the frame */
 	if (isa16) {
-		err = __put_user(ir >> 16,
-				 (u16 __user *)(&fr->emul));
-		err |= __put_user(ir & 0xffff,
-				  (u16 __user *)((long)(&fr->emul) + 2));
-		err |= __put_user(break_math >> 16,
-				  (u16 __user *)(&fr->badinst));
-		err |= __put_user(break_math & 0xffff,
-				  (u16 __user *)((long)(&fr->badinst) + 2));
+		union mips_instruction _emul = {
+			.halfword = { ir >> 16, ir }
+		};
+		union mips_instruction _badinst = {
+			.halfword = { break_math >> 16, break_math }
+		};
+
+		fr.emul = _emul.word;
+		fr.badinst = _badinst.word;
 	} else {
-		err = __put_user(ir, &fr->emul);
-		err |= __put_user(break_math, &fr->badinst);
+		fr.emul = ir;
+		fr.badinst = break_math;
 	}
 
-	if (unlikely(err)) {
+	/* Write the frame to user memory */
+	fr_uaddr = (unsigned long)&dsemul_page()[fr_idx];
+	ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr),
+				FOLL_FORCE | FOLL_WRITE);
+	if (unlikely(ret != sizeof(fr))) {
 		MIPS_FPU_EMU_INC_STATS(errors);
 		free_emuframe(fr_idx, current->mm);
 		return SIGBUS;
@@ -282,10 +287,7 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
 	atomic_set(&current->thread.bd_emu_frame, fr_idx);
 
 	/* Change user register context to execute the frame */
-	regs->cp0_epc = (unsigned long)&fr->emul | isa16;
-
-	/* Ensure the icache observes our newly written frame */
-	flush_cache_sigtramp((unsigned long)&fr->emul);
+	regs->cp0_epc = fr_uaddr | isa16;
 
 	return 0;
 }
-- 
2.20.0

Re: [PATCH] MIPS: math-emu: Write-protect delay slot emulation pages

[Paul Burton]

Hi Sasha,

On Thu, Dec 20, 2018 at 07:26:15PM +0000, Sasha Levin wrote:
> Hi,
> 
> [This is an automated email]
> 
> This commit has been processed because it contains a "Fixes:" tag,
> fixing commit: 432c6bacbd0c MIPS: Use per-mm page to execute branch delay slot instructions.
> 
> The bot has tested the following trees: v4.19.10, v4.14.89, v4.9.146, 

Neat! I like the idea of this automation :)

> v4.19.10: Build OK!
> v4.14.89: Build OK!
> v4.9.146: Failed to apply! Possible dependencies:
>     05ce77249d50 ("userfaultfd: non-cooperative: add madvise() event for MADV_DONTNEED request")
>     163e11bc4f6e ("userfaultfd: hugetlbfs: UFFD_FEATURE_MISSING_HUGETLBFS")
>     67dece7d4c58 ("x86/vdso: Set vDSO pointer only after success")
>     72f87654c696 ("userfaultfd: non-cooperative: add mremap() event")
>     893e26e61d04 ("userfaultfd: non-cooperative: Add fork() event")
>     897ab3e0c49e ("userfaultfd: non-cooperative: add event for memory unmaps")
>     9cd75c3cd4c3 ("userfaultfd: non-cooperative: add ability to report non-PF events from uffd descriptor")
>     d811914d8757 ("userfaultfd: non-cooperative: rename *EVENT_MADVDONTNEED to *EVENT_REMOVE")

This list includes the correct soft dependency - commit 897ab3e0c49e
("userfaultfd: non-cooperative: add event for memory unmaps") which
added an extra argument to mmap_region().

> How should we proceed with this patch?

The backport to v4.9 should simply drop the last argument (NULL) in the
call to mmap_region().

Is there some way I can indicate this sort of thing in future patches so
that the automation can spot that I already know it won't apply cleanly
to a particular range of kernel versions? Or even better, that I could
indicate what needs to be changed when backporting to those kernel
versions?

Thanks,
    Paul

Re: [PATCH] MIPS: math-emu: Write-protect delay slot emulation pages

[Sasha Levin]

On Fri, Dec 21, 2018 at 09:16:37PM +0000, Paul Burton wrote:
>Hi Sasha,
>
>On Thu, Dec 20, 2018 at 07:26:15PM +0000, Sasha Levin wrote:
>> Hi,
>>
>> [This is an automated email]
>>
>> This commit has been processed because it contains a "Fixes:" tag,
>> fixing commit: 432c6bacbd0c MIPS: Use per-mm page to execute branch delay slot instructions.
>>
>> The bot has tested the following trees: v4.19.10, v4.14.89, v4.9.146,
>
>Neat! I like the idea of this automation :)

Thank you! :)

>> v4.19.10: Build OK!
>> v4.14.89: Build OK!
>> v4.9.146: Failed to apply! Possible dependencies:
>>     05ce77249d50 ("userfaultfd: non-cooperative: add madvise() event for MADV_DONTNEED request")
>>     163e11bc4f6e ("userfaultfd: hugetlbfs: UFFD_FEATURE_MISSING_HUGETLBFS")
>>     67dece7d4c58 ("x86/vdso: Set vDSO pointer only after success")
>>     72f87654c696 ("userfaultfd: non-cooperative: add mremap() event")
>>     893e26e61d04 ("userfaultfd: non-cooperative: Add fork() event")
>>     897ab3e0c49e ("userfaultfd: non-cooperative: add event for memory unmaps")
>>     9cd75c3cd4c3 ("userfaultfd: non-cooperative: add ability to report non-PF events from uffd descriptor")
>>     d811914d8757 ("userfaultfd: non-cooperative: rename *EVENT_MADVDONTNEED to *EVENT_REMOVE")
>
>This list includes the correct soft dependency - commit 897ab3e0c49e
>("userfaultfd: non-cooperative: add event for memory unmaps") which
>added an extra argument to mmap_region().
>
>> How should we proceed with this patch?
>
>The backport to v4.9 should simply drop the last argument (NULL) in the
>call to mmap_region().
>
>Is there some way I can indicate this sort of thing in future patches so
>that the automation can spot that I already know it won't apply cleanly
>to a particular range of kernel versions? Or even better, that I could
>indicate what needs to be changed when backporting to those kernel
>versions?

The "official" way of doing that is described here:

	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/stable-kernel-rules.rst#n101

However, most people just either add a comment in the commit message, or
reply to the patch mail (or the "FAILED:" mail from Greg) saying how to
fix this. Either way works really.

Greg will also usually look up these automated mails when he adds stuff
to -stable, so if you describe how to deal with it here (like you did
above) is enough as well.

--
Thanks,
Sasha

Re: [PATCH] MIPS: math-emu: Write-protect delay slot emulation pages

[Paul Burton]

Hello,

Paul Burton wrote:
> Mapping the delay slot emulation page as both writeable & executable
> presents a security risk, in that if an exploit can write to & jump into
> the page then it can be used as an easy way to execute arbitrary code.
> 
> Prevent this by mapping the page read-only for userland, and using
> access_process_vm() with the FOLL_FORCE flag to write to it from
> mips_dsemul().
> 
> This will likely be less efficient due to copy_to_user_page() performing
> cache maintenance on a whole page, rather than a single line as in the
> previous use of flush_cache_sigtramp(). However this delay slot
> emulation code ought not to be running in any performance critical paths
> anyway so this isn't really a problem, and we can probably do better in
> copy_to_user_page() anyway in future.
> 
> A major advantage of this approach is that the fix is small & simple to
> backport to stable kernels.
> 
> Reported-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Paul Burton <paul.burton@mips.com>
> Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")
> Cc: stable@vger.kernel.org # v4.8+

Applied to mips-fixes.

Thanks,
    Paul

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