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Date:   Mon, 27 Apr 2020 15:09:42 -0700
From:   Sami Tolvanen <samitolvanen@google.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Marc Zyngier <maz@kernel.org>,
        kernel-hardening@lists.openwall.com,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Laura Abbott <labbott@redhat.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v13 00/12] add support for Clang's Shadow Call Stack
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On Mon, Apr 27, 2020 at 10:50:34PM +0200, Ard Biesheuvel wrote:
> > OK, so one thing that came up in an offline discussion about SCS is
> > the way it interacts with the vmap'ed stack.
> >
> > The vmap'ed stack is great for robustness, but it only works if things
> > don't explode for other reasons in the mean time. This means the
> > ordinary-to-shadow-call-stack size ratio should be chosen such that it
> > is *really* unlikely you could ever overflow the shadow call stack and
> > corrupt another task's call stack before hitting the vmap stack's
> > guard region.
> >
> > Alternatively, I wonder if there is a way we could let the SCS and
> > ordinary stack share the [bottom of] the vmap'ed region. That would
> > give rather nasty results if the ordinary stack overflows into the
> > SCS, but for cases where we really recurse out of control, we could
> > catch this occurrence on either stack, whichever one occurs first. And
> > the nastiness -when it does occur- will not corrupt any state beyond
> > the stack of the current task.
> 
> Hmm, I guess that would make it quite hard to keep the SCS address
> secret though :-(

Yes, and the stack potentially overflowing into the SCS sort of defeats
the purpose. I'm fine with increasing the SCS size to something safer,
but using a vmapped shadow stack seems like the correct solution to this
problem, at least on devices where allocating a full page isn't an issue.

Sami