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

Will Deacon
2020-02-10T16:41:47Z

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@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..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@kernel.org>

Will