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

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Message-ID: <ZRWw7aa3C0LlMPTH@arm.com>
Date:   Thu, 28 Sep 2023 17:59:25 +0100
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Mark Brown <broonie@...nel.org>, 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

The 08/23/2023 14:11, Catalin Marinas wrote:
> On Wed, Aug 23, 2023 at 11:09:59AM +0100, Szabolcs Nagy wrote:
> > The 08/22/2023 18:53, Mark Brown wrote:
> > > I do worry about the story for users calling the underlying clone3() API
> > > (or legacy clone() for that matter) directly, and we would also need to
> > > handle the initial GCS enable via prctl() - that's not insurmountable,
> > > we could add a size argument there that only gets interpreted during the
> > > initial enable for example.
...
> > and there is user code doing raw clone threads (such threads are
> > technically not allowed to call into libc) it's not immediately
> > clear to me if having gcs in those threads is better or worse.

i think raw clone / clone3 users may be relevant so we need a
solution such that they don't fail when gcs args are missing.

userspace allocated gcs works for me, but maybe the alternative
with size only is more consistent (thread gcs is kernel mapped
with fallback size logic if gcs size is missing):

> An alternative would be for the clone3() to provide an address _hint_
> and size for GCS and it would still be the kernel doing the mmap (and
> munmap on clearing). But at least the user has some control over the
> placement of the GCS and its size (and maybe providing the address has
> MAP_FIXED semantics).

the main thread gcs is still special: the size is provided
via prctl (if at all).