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

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Message-ID: <CABCJKufpgoqo84GvV42bO-LVPZ4morV=OhscTNwaBpv-RSwXUw@mail.gmail.com>
Date:   Tue, 5 Nov 2019 14:05:39 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Mark Rutland <mark.rutland@....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 <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 11/17] arm64: disable function graph tracing with SCS

On Tue, Nov 5, 2019 at 11:55 AM Mark Rutland <mark.rutland@....com> wrote:
> On Mon, Nov 04, 2019 at 03:44:39PM -0800, Sami Tolvanen wrote:
> > Sure, I'll add a better description in v5. In this case, the return
> > address is modified in the kernel stack, which means the changes are
> > ignored with SCS.
>
> Ok, that makes sense to me. I'd suggest something like:
>
> | 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 the graph tracer when SCS is
> | enabled.
>
> ... as I suspect with some rework of the trampoline and common ftrace
> code we'd be able to correctly manipulate the shadow stack for this.
> Similarly, if clang gained -fpatchable-funciton-etnry, we'd get that for
> free.

That sounds good to me. Thanks, Mark.

Sami