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

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Message-ID: <ZOTnL1SDJWZjHPUW@arm.com>
Date:   Tue, 22 Aug 2023 17:49:51 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Mark Brown <broonie@...nel.org>
Cc:     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>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        "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 Fri, Aug 18, 2023 at 08:38:02PM +0100, Mark Brown wrote:
> On Fri, Aug 18, 2023 at 06:29:54PM +0100, Catalin Marinas wrote:
> > A related question - it may have been discussed intensively on the x86
> > thread (I may read it sometime) - why not have the libc map the shadow
> 
> Your assumption that this is a single thread feels optimistic there.

Yeah and I unfortunately ignored all of them.

> > stack and pass the pointer/size to clone3()? It saves us from having to
> > guess what the right size we'd need. struct clone_args is extensible.
> 
> I can't recall or locate the specific reasoning there right now, perhaps
> Rick or someone else can?  I'd guess there would be compat concerns for
> things that don't go via libc which would complicate the story with
> identifying and marking things as GCS/SS safe, it's going to be more
> robust to just supply a GCS if the process is using it.  That said
> having a default doesn't preclude us using the extensibility to allow
> userspace directly to control the GCS size, I would certainly be in
> favour of adding support for that.

It would be good if someone provided a summary of the x86 decision (I'll
get to those thread but most likely in September). I think we concluded
that we can't deploy GCS entirely transparently, so we need a libc
change (apart from the ELF annotations). Since libc is opting in to GCS,
we could also update the pthread_create() etc. to allocate the shadow
together with the standard stack.

Anyway, that's my preference but maybe there were good reasons not to do
this.

-- 
Catalin