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

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Message-ID: <ZOd6lzj29VksAp7L@arm.com>
Date:   Thu, 24 Aug 2023 16:43:19 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Mark Brown <broonie@...nel.org>
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 Wed, Aug 23, 2023 at 07:16:52PM +0100, Mark Brown wrote:
> On Wed, Aug 23, 2023 at 06:40:40PM +0100, Szabolcs Nagy wrote:
> > i don't know if we can allow disabled gcs thread creation with locked
> > gcs state. (i can see arguments both ways, so further prctl flag may
> > be needed which may be another divergence from x86)
> 
> I think that if we do add a new flag that'd just be new functionality,
> the divergence would be in allowing configuration via clone3() rather
> than the flag.  TBH I'm not sure I see a use case for locking but
> providing a mechanism for getting out of the lock, that seems very
> questionable.

You are right, once the configuration is locked a plain clone() or
clone3() without a GCS pointer should be rejected.

Is there a use-case for the unlocked configuration to allow disabling
the GCS implicitly via a clone syscall?

If we go for extending clone3, I wonder whether we should also introduce
a sigaltstack2/3 ;). I haven't checked what the current patches do and
won't have time until early September (on holiday from the end of today).

-- 
Catalin