linux-kernel - Re: [PATCH v7 04/11] scs: disable when function graph tracing is enabled

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Message-ID: <202001281449.FB1671805E@keescook>
Date:   Tue, 28 Jan 2020 14:50:00 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <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>,
        Mark Rutland <mark.rutland@....com>, james.morse@....com,
        Dave Martin <Dave.Martin@....com>,
        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 v7 04/11] scs: disable when function graph tracing is
 enabled

On Tue, Jan 28, 2020 at 10:49:27AM -0800, Sami Tolvanen wrote:
> The graph tracer hooks returns by modifying frame records on the
> (regular) stack, but with SCS the return address is taken from the
> shadow stack, and the value in the frame record has no effect. As we
> don't currently have a mechanism to determine the corresponding slot
> on the shadow stack (and to pass this through the ftrace
> infrastructure), for now let's disable SCS when the graph tracer is
> enabled.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

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

-Kees

> ---
>  arch/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 1b16aa9a3fe5..0d746373c52e 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -530,6 +530,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
>  
>  config SHADOW_CALL_STACK
>  	bool "Clang Shadow Call Stack"
> +	depends on !FUNCTION_GRAPH_TRACER
>  	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
>  	help
>  	  This option enables Clang's Shadow Call Stack, which uses a
> -- 
> 2.25.0.341.g760bfbb309-goog
> 

-- 
Kees Cook