linux-kernel - Re: [PATCH v4 11/17] arm64: disable function graph tracing with SCS

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Message-ID: <20191104171132.GB2024@lakrids.cambridge.arm.com>
Date:   Mon, 4 Nov 2019 17:11:33 +0000
From:   Mark Rutland <mark.rutland@....com>
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>,
        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 v4 11/17] arm64: disable function graph tracing with SCS

On Fri, Nov 01, 2019 at 03:11:44PM -0700, Sami Tolvanen wrote:
> With CONFIG_FUNCTION_GRAPH_TRACER, function return addresses are
> modified in ftrace_graph_caller and prepare_ftrace_return to redirect
> control flow to ftrace_return_to_handler. This is incompatible with
> SCS.

Can you please elaborate on _how_ this is incompatible in the commit
message?

For example, it's not clear to me if you mean that's functionally
incompatible, or if you're trying to remove return-altering gadgets.

If there's a functional incompatibility, please spell that out a bit
more clearly. Likewise if this is about minimizing the set of places
that can mess with control-flow outside of usual function conventions.

Thanks,
Mark.

> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e7b57a8a5531..42867174920f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -148,7 +148,7 @@ config ARM64
>  	select HAVE_FTRACE_MCOUNT_RECORD
>  	select HAVE_FUNCTION_TRACER
>  	select HAVE_FUNCTION_ERROR_INJECTION
> -	select HAVE_FUNCTION_GRAPH_TRACER
> +	select HAVE_FUNCTION_GRAPH_TRACER if !SHADOW_CALL_STACK
>  	select HAVE_GCC_PLUGINS
>  	select HAVE_HW_BREAKPOINT if PERF_EVENTS
>  	select HAVE_IRQ_TIME_ACCOUNTING
> -- 
> 2.24.0.rc1.363.gb1bccd3e3d-goog
>