linux-kernel - Re: [PATCH v7 20/39] arm64/gcs: Context switch GCS state for EL0

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Message-ID: <b2a717a7-e6d3-4fa9-921e-156c17ad5424@sirena.org.uk>
Date:   Wed, 13 Dec 2023 20:02:24 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Deepak Gupta <debug@...osinc.com>
Cc:     Catalin Marinas <catalin.marinas@....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>,
        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>,
        Florian Weimer <fweimer@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Thiago Jung Bauermann <thiago.bauermann@...aro.org>,
        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 v7 20/39] arm64/gcs: Context switch GCS state for EL0

On Wed, Dec 13, 2023 at 11:59:45AM -0800, Deepak Gupta wrote:
> On Wed, Nov 22, 2023 at 1:45 AM Mark Brown <broonie@...nel.org> wrote:

> > +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_ENABLE)
> > +               gcscre0_el1 |= GCSCRE0_EL1_RVCHKEN | GCSCRE0_EL1_PCRSEL;

> If the intent is to disable, is the GCS stack freed or kept around?
> I expect if libc is taking the decision to disable, kernel should free it up.
> Is it freed in some other flow?

Kept around and freed on thread exit.  There is a potential race between
for example disabling in a signal handler and something trying to walk
the stack so we err on the side of caution.

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