Re: [PATCH v4 10/17] arm64: disable kretprobes with SCS

[Mark Rutland]

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@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  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
>