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Date: Thu, 16 Jan 2020 17:44:51 +0000
From: Will Deacon <will@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Dave Martin <Dave.Martin@....com>,
	Kees Cook <keescook@...omium.org>,
	Laura Abbott <labbott@...hat.com>, Marc Zyngier <maz@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Jann Horn <jannh@...gle.com>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	clang-built-linux@...glegroups.com,
	kernel-hardening@...ts.openwall.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 11/15] arm64: efi: restore x18 if it was corrupted

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@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.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

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