linux-kernel - Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks

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Message-ID: <638a7be5-6662-471d-a3ce-0de0ac768e99@sirena.org.uk>
Date:   Fri, 6 Oct 2023 13:17:01 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Will Deacon <will@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        "Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
        Deepak Gupta <debug@...osinc.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control
 Stacks

On Thu, Oct 05, 2023 at 06:23:10PM +0100, Catalin Marinas wrote:

> It's not just the default size that I dislike (I think the x86
> RLIMIT_STACK or clone3() stack_size is probably good enough) but the
> kernel allocating the shadow stack and inserting it into the user
> address space. The actual thread stack is managed by the user but the
> shadow stack is not (and we don't do this very often). Anyway, I don't
> have a better solution for direct uses of clone() or clone3(), other
> than running those threads with the shadow stack disabled. Not sure
> that's desirable.

Running threads with the shadow stack disabled if they don't explicitly
request it feels like it's asking for trouble - as well as the escape
route from the protection it'd provide I'd expect there to be trouble
for things that do stack pivots, potentially random issues if there's a
mix of ways threads are started.  It's going to be a tradeoff whatever
we do.

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