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

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Date:   Mon, 21 Oct 2019 08:20:43 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/18] arm64: efi: restore x18 if it was corrupted

On Fri, 18 Oct 2019 at 18:11, Sami Tolvanen <samitolvanen@...gle.com> 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@...gle.com>
> ---
>  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
>