Re: [PATCH v6 11/15] arm64: efi: restore x18 if it was corrupted

Will Deacon
2020-01-16T17:45:16Z

On Fri, Dec 06, 2019 at 02:13:47PM -0800, Sami Tolvanen 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@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
> 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..62f0260f5c17 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:
> +#ifdef CONFIG_SHADOW_CALL_STACK
> + /*
> + * Restore x18 before returning to 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
> +#endif

Why not restore it regardless of CONFIG_SHADOW_CALL_STACK?

Will