[v4,10/17] arm64: disable kretprobes with SCS
diff mbox series

Message ID 20191101221150.116536-11-samitolvanen@google.com
State New
Headers show
Series
  • add support for Clang's Shadow Call Stack
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Commit Message

Sami Tolvanen Nov. 1, 2019, 10:11 p.m. UTC
With CONFIG_KRETPROBES, function return addresses are modified to
redirect control flow to kretprobe_trampoline. This is incompatible
with SCS.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 arch/arm64/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Mark Rutland Nov. 4, 2019, 5:04 p.m. UTC | #1
On Fri, Nov 01, 2019 at 03:11:43PM -0700, Sami Tolvanen wrote:
> With CONFIG_KRETPROBES, function return addresses are modified to
> redirect control flow to kretprobe_trampoline. This is incompatible
> with SCS.

I'm a bit confused as to why that's the case -- could you please
elaborate on how this is incompatible?

IIUC kretrobes works by patching the function entry point with a BRK, so
that it can modify the LR _before_ it is saved to the stack. I don't see
how SCS affects that.

When the instrumented function returns, it'll balance its SCS state,
then "return" to kretprobe_trampoline. Since kretprobe_trampoline is
plain assembly, it doesn't have SCS, and can modify the LR live, as it
does.

So functionally, that appears to work. What am I missing? 

Thanks,
Mark.

> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 3f047afb982c..e7b57a8a5531 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -165,7 +165,7 @@ config ARM64
>  	select HAVE_STACKPROTECTOR
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_KPROBES
> -	select HAVE_KRETPROBES
> +	select HAVE_KRETPROBES if !SHADOW_CALL_STACK
>  	select HAVE_GENERIC_VDSO
>  	select IOMMU_DMA if IOMMU_SUPPORT
>  	select IRQ_DOMAIN
> -- 
> 2.24.0.rc1.363.gb1bccd3e3d-goog
>
Sami Tolvanen Nov. 4, 2019, 11:42 p.m. UTC | #2
On Mon, Nov 4, 2019 at 9:05 AM Mark Rutland <mark.rutland@arm.com> wrote:
> I'm a bit confused as to why that's the case -- could you please
> elaborate on how this is incompatible?
>
> IIUC kretrobes works by patching the function entry point with a BRK, so
> that it can modify the LR _before_ it is saved to the stack. I don't see
> how SCS affects that.

You're correct. While this may not be optimal for reducing attack
surface, I just tested this to confirm that there's no functional
conflict. I'll drop this and related patches from v5.

Sami
Mark Rutland Nov. 5, 2019, 9:04 a.m. UTC | #3
On Mon, Nov 04, 2019 at 03:42:09PM -0800, Sami Tolvanen wrote:
> On Mon, Nov 4, 2019 at 9:05 AM Mark Rutland <mark.rutland@arm.com> wrote:
> > I'm a bit confused as to why that's the case -- could you please
> > elaborate on how this is incompatible?
> >
> > IIUC kretrobes works by patching the function entry point with a BRK, so
> > that it can modify the LR _before_ it is saved to the stack. I don't see
> > how SCS affects that.
> 
> You're correct. While this may not be optimal for reducing attack
> surface, I just tested this to confirm that there's no functional
> conflict. I'll drop this and related patches from v5.

Great; thanks for confirming!

Mark.

Patch
diff mbox series

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 3f047afb982c..e7b57a8a5531 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -165,7 +165,7 @@  config ARM64
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_KPROBES
-	select HAVE_KRETPROBES
+	select HAVE_KRETPROBES if !SHADOW_CALL_STACK
 	select HAVE_GENERIC_VDSO
 	select IOMMU_DMA if IOMMU_SUPPORT
 	select IRQ_DOMAIN