diff mbox

[PATCHv2] arm64: kasan: clear stale stack poison

Message ID 1456862465-31505-1-git-send-email-mark.rutland@arm.com
State New
Headers show

Commit Message

Mark Rutland March 1, 2016, 8:01 p.m. UTC 
When a CPU is shut down or placed into a low power state, the functions
on the critical path to firmware never return, and hence their epilogues
never execute. When using KASAN, this means that the shadow entries for
the corresponding stack are poisoned but never unpoisoned. When a CPU
subsequently re-enters the kernel via another path, and begins using
the stack, it may hit stale poison values, leading to false-positive
KASAN failures.

We can't ensure that all functions on the critical path are not
instrumented. For CPU hotplug this includes lots of core code starting
from secondary_start_kernel, and for CPU idle we can't ensure that
specific functions are not instrumented, as the compiler always poisons
the stack even when told to not instrument a function:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69863

This patch works around the issue by forcefully unpoisoning the shadow
region for all stack on the critical path, before we return to
instrumented C code. As we cannot statically determine the stack usage
of code in the critical path, we must clear the shadow for all remaining
stack, meaning that we must clear up to 2K of shadow memory each time a
CPU enters the kernel from idle or hotplug.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>

Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
---
 arch/arm64/include/asm/kasan.h  | 40 ++++++++++++++++++++++++++++++++++------
 arch/arm64/kernel/asm-offsets.c |  1 +
 arch/arm64/kernel/head.S        |  2 ++
 arch/arm64/kernel/sleep.S       |  2 ++
 4 files changed, 39 insertions(+), 6 deletions(-)

Since v1 [1]:
* Remove unneeded offset
* Simplify calculation

For the timebeing I've retained the arm64-specific hotplug fix, though it might
be possible to handle the hotplug case in common code (e.g. [2]). 

Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/409466.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/412923.html

-- 
1.9.1


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Comments

Mark Rutland March 2, 2016, 11:48 a.m. UTC | #1 
On Wed, Mar 02, 2016 at 10:56:20AM +0000, Lorenzo Pieralisi wrote:
> On Tue, Mar 01, 2016 at 08:01:05PM +0000, Mark Rutland wrote:

> 

> [...]

> 

> > diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h

> > index 2774fa3..6f00b76 100644

> > --- a/arch/arm64/include/asm/kasan.h

> > +++ b/arch/arm64/include/asm/kasan.h

> > @@ -1,10 +1,30 @@

> >  #ifndef __ASM_KASAN_H

> >  #define __ASM_KASAN_H

> >  

> > -#ifndef __ASSEMBLY__

> > -

> > +#ifndef LINKER_SCRIPT

> >  #ifdef CONFIG_KASAN

> >  

> > +#ifdef __ASSEMBLY__

> > +

> > +#include <asm/asm-offsets.h>

> > +#include <asm/thread_info.h>

> > +

> > +	/*

> > +	 * Remove stale shadow posion for the stack left over from a prior

> > +	 * hot-unplug or idle exit, from the lowest stack address in the

> > +	 * thread_union up to the covering up to the current stack pointer.

> > +	 * Shadow poison above this is preserved.

> > +	 */

> > +	.macro kasan_unpoison_stack

> > +	mov	x1, sp

> > +	and	x0, x1, #~(THREAD_SIZE - 1)

> 

> I suspect you did not use sp_el0 on purpose here (that contains a

> pointer to thread_info), just asking.


I worked on the assumption that the arithmetic was likely to be faster
than a system register access, but I do not have numbers to back that
up.

I'm happy to use sp_el0 if that's preferrable.

> > +	add	x0, x0, #(THREAD_INFO_SIZE)

> > +	sub	x1, x1, x0

> > +	bl	kasan_unpoison_shadow

> 

> I wonder whether a wrapper function eg kasan_unpoison_stack(addr) is

> better, where the thread info/stack address computation can be done in C

> we just pass it the precise bottom of the stack location watermark, which

> is the only reason why we want to call it from assembly.


True; I'll have a go.

> Other than that:

> 

> Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>


Cheers!

Mark.

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Catalin Marinas March 2, 2016, 12:47 p.m. UTC | #2 
On Wed, Mar 02, 2016 at 11:48:34AM +0000, Mark Rutland wrote:
> On Wed, Mar 02, 2016 at 10:56:20AM +0000, Lorenzo Pieralisi wrote:

> > On Tue, Mar 01, 2016 at 08:01:05PM +0000, Mark Rutland wrote:

> > 

> > [...]

> > 

> > > diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h

> > > index 2774fa3..6f00b76 100644

> > > --- a/arch/arm64/include/asm/kasan.h

> > > +++ b/arch/arm64/include/asm/kasan.h

> > > @@ -1,10 +1,30 @@

> > >  #ifndef __ASM_KASAN_H

> > >  #define __ASM_KASAN_H

> > >  

> > > -#ifndef __ASSEMBLY__

> > > -

> > > +#ifndef LINKER_SCRIPT

> > >  #ifdef CONFIG_KASAN

> > >  

> > > +#ifdef __ASSEMBLY__

> > > +

> > > +#include <asm/asm-offsets.h>

> > > +#include <asm/thread_info.h>

> > > +

> > > +	/*

> > > +	 * Remove stale shadow posion for the stack left over from a prior

> > > +	 * hot-unplug or idle exit, from the lowest stack address in the

> > > +	 * thread_union up to the covering up to the current stack pointer.

> > > +	 * Shadow poison above this is preserved.

> > > +	 */

> > > +	.macro kasan_unpoison_stack

> > > +	mov	x1, sp

> > > +	and	x0, x1, #~(THREAD_SIZE - 1)

> > 

> > I suspect you did not use sp_el0 on purpose here (that contains a

> > pointer to thread_info), just asking.

> 

> I worked on the assumption that the arithmetic was likely to be faster

> than a system register access, but I do not have numbers to back that

> up.

> 

> I'm happy to use sp_el0 if that's preferrable.


Since we need sp anyway, it is probably faster. But, I think the current
patch is more to the point - clearing from the bottom of the current
stack up to the current sp, we don't need any assumption about sp_el0 (I
guess we never call cpu_suspend() on an IRQ stack, though I'm not 100%
sure).

I don't think Will wants to send this as a fix for 4.5, probably not
urgent but I'll let him decide. In any case:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>


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Will Deacon March 2, 2016, 1:55 p.m. UTC | #3 
On Wed, Mar 02, 2016 at 12:47:24PM +0000, Catalin Marinas wrote:
> On Wed, Mar 02, 2016 at 11:48:34AM +0000, Mark Rutland wrote:

> > On Wed, Mar 02, 2016 at 10:56:20AM +0000, Lorenzo Pieralisi wrote:

> > > On Tue, Mar 01, 2016 at 08:01:05PM +0000, Mark Rutland wrote:

> > > 

> > > [...]

> > > 

> > > > diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h

> > > > index 2774fa3..6f00b76 100644

> > > > --- a/arch/arm64/include/asm/kasan.h

> > > > +++ b/arch/arm64/include/asm/kasan.h

> > > > @@ -1,10 +1,30 @@

> > > >  #ifndef __ASM_KASAN_H

> > > >  #define __ASM_KASAN_H

> > > >  

> > > > -#ifndef __ASSEMBLY__

> > > > -

> > > > +#ifndef LINKER_SCRIPT

> > > >  #ifdef CONFIG_KASAN

> > > >  

> > > > +#ifdef __ASSEMBLY__

> > > > +

> > > > +#include <asm/asm-offsets.h>

> > > > +#include <asm/thread_info.h>

> > > > +

> > > > +	/*

> > > > +	 * Remove stale shadow posion for the stack left over from a prior

> > > > +	 * hot-unplug or idle exit, from the lowest stack address in the

> > > > +	 * thread_union up to the covering up to the current stack pointer.

> > > > +	 * Shadow poison above this is preserved.

> > > > +	 */

> > > > +	.macro kasan_unpoison_stack

> > > > +	mov	x1, sp

> > > > +	and	x0, x1, #~(THREAD_SIZE - 1)

> > > 

> > > I suspect you did not use sp_el0 on purpose here (that contains a

> > > pointer to thread_info), just asking.

> > 

> > I worked on the assumption that the arithmetic was likely to be faster

> > than a system register access, but I do not have numbers to back that

> > up.

> > 

> > I'm happy to use sp_el0 if that's preferrable.

> 

> Since we need sp anyway, it is probably faster. But, I think the current

> patch is more to the point - clearing from the bottom of the current

> stack up to the current sp, we don't need any assumption about sp_el0 (I

> guess we never call cpu_suspend() on an IRQ stack, though I'm not 100%

> sure).

> 

> I don't think Will wants to send this as a fix for 4.5, probably not

> urgent but I'll let him decide. In any case:

> 

> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>


I wasn't rushing to queue this, especially with the hotplug case
outstanding. Feel free to take it for 4.6.

Will

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diff mbox

Patch

diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
index 2774fa3..6f00b76 100644
--- a/arch/arm64/include/asm/kasan.h
+++ b/arch/arm64/include/asm/kasan.h
@@ -1,10 +1,30 @@ 
 #ifndef __ASM_KASAN_H
 #define __ASM_KASAN_H
 
-#ifndef __ASSEMBLY__
-
+#ifndef LINKER_SCRIPT
 #ifdef CONFIG_KASAN
 
+#ifdef __ASSEMBLY__
+
+#include <asm/asm-offsets.h>
+#include <asm/thread_info.h>
+
+	/*
+	 * Remove stale shadow posion for the stack left over from a prior
+	 * hot-unplug or idle exit, from the lowest stack address in the
+	 * thread_union up to the covering up to the current stack pointer.
+	 * Shadow poison above this is preserved.
+	 */
+	.macro kasan_unpoison_stack
+	mov	x1, sp
+	and	x0, x1, #~(THREAD_SIZE - 1)
+	add	x0, x0, #(THREAD_INFO_SIZE)
+	sub	x1, x1, x0
+	bl	kasan_unpoison_shadow
+	.endm
+
+#else /* __ASSEMBLY__ */
+
 #include <linux/linkage.h>
 #include <asm/memory.h>
 
@@ -30,9 +50,17 @@ 
 void kasan_init(void);
 asmlinkage void kasan_early_init(void);
 
-#else
+#endif /* __ASSEMBLY__ */
+
+#else /* CONFIG_KASAN */
+
+#ifdef __ASSEMBLY__
+	.macro kasan_unpoison_stack
+	.endm
+#else /* __ASSEMBLY */
 static inline void kasan_init(void) { }
-#endif
+#endif /* __ASSEMBLY__ */
 
-#endif
-#endif
+#endif /* CONFIG_KASAN */
+#endif /* LINKER_SCRIPT */
+#endif /* __ASM_KASAN_H */
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index fffa4ac6..c615fa3 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -39,6 +39,7 @@  int main(void)
   DEFINE(TI_ADDR_LIMIT,		offsetof(struct thread_info, addr_limit));
   DEFINE(TI_TASK,		offsetof(struct thread_info, task));
   DEFINE(TI_CPU,		offsetof(struct thread_info, cpu));
+  DEFINE(THREAD_INFO_SIZE,	sizeof(struct thread_info));
   BLANK();
   DEFINE(THREAD_CPU_CONTEXT,	offsetof(struct task_struct, thread.cpu_context));
   BLANK();
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 917d981..35ae2cb 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -29,6 +29,7 @@ 
 #include <asm/asm-offsets.h>
 #include <asm/cache.h>
 #include <asm/cputype.h>
+#include <asm/kasan.h>
 #include <asm/kernel-pgtable.h>
 #include <asm/memory.h>
 #include <asm/pgtable-hwdef.h>
@@ -616,6 +617,7 @@  ENTRY(__secondary_switched)
 	and	x0, x0, #~(THREAD_SIZE - 1)
 	msr	sp_el0, x0			// save thread_info
 	mov	x29, #0
+	kasan_unpoison_stack
 	b	secondary_start_kernel
 ENDPROC(__secondary_switched)
 
diff --git a/arch/arm64/kernel/sleep.S b/arch/arm64/kernel/sleep.S
index e33fe33..3ad7681 100644
--- a/arch/arm64/kernel/sleep.S
+++ b/arch/arm64/kernel/sleep.S
@@ -2,6 +2,7 @@ 
 #include <linux/linkage.h>
 #include <asm/asm-offsets.h>
 #include <asm/assembler.h>
+#include <asm/kasan.h>
 
 	.text
 /*
@@ -145,6 +146,7 @@  ENTRY(cpu_resume_mmu)
 ENDPROC(cpu_resume_mmu)
 	.popsection
 cpu_resume_after_mmu:
+	kasan_unpoison_stack
 	mov	x0, #0			// return zero on success
 	ldp	x19, x20, [sp, #16]
 	ldp	x21, x22, [sp, #32]