Re: [PATCH v3 14/17] arm64: efi: restore x18 if it was corrupted

[Kees Cook]

On Thu, Oct 31, 2019 at 09:46:34AM -0700, samitolvanen@xxxxxxxxxx wrote:
> If we detect a corrupted x18 and SCS is enabled, restore the register
> before jumping back to instrumented code. This is safe, because the
> wrapper is called with preemption disabled and a separate shadow stack
> is used for interrupt handling.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/arm64/kernel/efi-rt-wrapper.S | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/efi-rt-wrapper.S b/arch/arm64/kernel/efi-rt-wrapper.S
> index 3fc71106cb2b..945744f16086 100644
> --- a/arch/arm64/kernel/efi-rt-wrapper.S
> +++ b/arch/arm64/kernel/efi-rt-wrapper.S
> @@ -34,5 +34,10 @@ ENTRY(__efi_rt_asm_wrapper)
>  	ldp	x29, x30, [sp], #32
>  	b.ne	0f
>  	ret
> -0:	b	efi_handle_corrupted_x18	// tail call
> +0:
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +	/* Restore x18 before returning to instrumented code. */
> +	mov	x18, x2
> +#endif
> +	b	efi_handle_corrupted_x18	// tail call
>  ENDPROC(__efi_rt_asm_wrapper)
> -- 
> 2.24.0.rc0.303.g954a862665-goog
> 

-- 
Kees Cook