Re: [PATCH 14/18] arm64: efi: restore x18 if it was corrupted

[Ard Biesheuvel]

On Fri, 18 Oct 2019 at 18:11, Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
>
> If we detect a corrupted x18 and SCS is enabled, restore the register
> before jumping back to instrumented code.
>

You'll have to elaborate a bit here and explain that this is
sufficient, given that we run EFI runtime services with interrupts
enabled.

> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> ---
>  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)
> --
> 2.23.0.866.gb869b98d4c-goog
>