linux-kernel - Re: [PATCH v3 04/17] arm64: kernel: avoid x18 __cpu_soft

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Message-ID: <201910312046.53A31D394@keescook>
Date:   Thu, 31 Oct 2019 20:47:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     samitolvanen@...gle.com
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        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 v3 04/17] arm64: kernel: avoid x18 __cpu_soft_restart

On Thu, Oct 31, 2019 at 09:46:24AM -0700, samitolvanen@...gle.com wrote:
> From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> 
> The code in __cpu_soft_restart() uses x18 as an arbitrary temp register,
> which will shortly be disallowed. So use x8 instead.
> 
> Link: https://patchwork.kernel.org/patch/9836877/
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>

This one is easy to validate! ;)

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

-Kees

> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Mark Rutland <mark.rutland@....com>
> ---
>  arch/arm64/kernel/cpu-reset.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpu-reset.S b/arch/arm64/kernel/cpu-reset.S
> index 6ea337d464c4..32c7bf858dd9 100644
> --- a/arch/arm64/kernel/cpu-reset.S
> +++ b/arch/arm64/kernel/cpu-reset.S
> @@ -42,11 +42,11 @@ ENTRY(__cpu_soft_restart)
>  	mov	x0, #HVC_SOFT_RESTART
>  	hvc	#0				// no return
>  
> -1:	mov	x18, x1				// entry
> +1:	mov	x8, x1				// entry
>  	mov	x0, x2				// arg0
>  	mov	x1, x3				// arg1
>  	mov	x2, x4				// arg2
> -	br	x18
> +	br	x8
>  ENDPROC(__cpu_soft_restart)
>  
>  .popsection
> -- 
> 2.24.0.rc0.303.g954a862665-goog
> 

-- 
Kees Cook