Re: [PATCH v7 07/11] arm64: efi: restore x18 if it was corrupted

[Will Deacon]

On Tue, Jan 28, 2020 at 10:49:30AM -0800, Sami Tolvanen wrote:
> If we detect a corrupted x18, restore the register before jumping back
> to potentially SCS 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>
> ---
>  arch/arm64/kernel/efi-rt-wrapper.S | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/efi-rt-wrapper.S b/arch/arm64/kernel/efi-rt-wrapper.S
> index 3fc71106cb2b..6ca6c0dc11a1 100644
> --- a/arch/arm64/kernel/efi-rt-wrapper.S
> +++ b/arch/arm64/kernel/efi-rt-wrapper.S
> @@ -34,5 +34,14 @@ ENTRY(__efi_rt_asm_wrapper)
>  	ldp	x29, x30, [sp], #32
>  	b.ne	0f
>  	ret
> -0:	b	efi_handle_corrupted_x18	// tail call
> +0:
> +	/*
> +	 * With CONFIG_SHADOW_CALL_STACK, the kernel uses x18 to store a
> +	 * shadow stack pointer, which we need to restore before returning to
> +	 * potentially instrumented code. This is safe because the wrapper is
> +	 * called with preemption disabled and a separate shadow stack is used
> +	 * for interrupts.
> +	 */
> +	mov	x18, x2
> +	b	efi_handle_corrupted_x18	// tail call
>  ENDPROC(__efi_rt_asm_wrapper)

Acked-by: Will Deacon <will@xxxxxxxxxx>

Will