linux-kernel - Re: [PATCH v11 04/12] scs: disable when function graph tracing is enabled

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Message-ID: <20200417231937.GA214321@google.com>
Date:   Fri, 17 Apr 2020 16:19:37 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.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 v11 04/12] scs: disable when function graph tracing is
 enabled

On Fri, Apr 17, 2020 at 04:46:14PM +0100, Mark Rutland wrote:
> If KRETPROBES works by messing with the LR at the instnat the function
> is entered, that should work similarly. If it works by replacing the
> RET it should also work out since any pauth/SCS work will have been
> undone by that point. If it attempts to mess with the return address in
> the middle of a function then it's not reliable today.

I did initially have a patch to disable kretprobes (until v5), but as
Mark pointed out back then, the return address is modified before it
gets pushed to the shadow stack, so there was no conflict with SCS. I
confirmed this on arm64, but haven't looked at other architectures.

Sami