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Date:   Thu, 16 Jan 2020 17:44:51 +0000
From:   Will Deacon <will@kernel.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux@googlegroups.com,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 11/15] arm64: efi: restore x18 if it was corrupted
Message-ID: <20200116174450.GD21396@willie-the-truck>
References: <20191018161033.261971-1-samitolvanen@google.com>
 <20191206221351.38241-1-samitolvanen@google.com>
 <20191206221351.38241-12-samitolvanen@google.com>
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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