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

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Date:   Wed, 13 Dec 2023 11:59:45 -0800
From:   Deepak Gupta <debug@...osinc.com>
To:     Mark Brown <broonie@...nel.org>
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, Nov 22, 2023 at 1:45 AM Mark Brown <broonie@...nel.org> wrote:
> +
> +/*
> + * Apply the GCS mode configured for the specified task to the
> + * hardware.
> + */
> +void gcs_set_el0_mode(struct task_struct *task)
> +{
> +       u64 gcscre0_el1 = GCSCRE0_EL1_nTR;
> +
> +       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?

> +
> +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_WRITE)
> +               gcscre0_el1 |= GCSCRE0_EL1_STREn;
> +
> +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_PUSH)
> +               gcscre0_el1 |= GCSCRE0_EL1_PUSHMEn;
> +
> +       write_sysreg_s(gcscre0_el1, SYS_GCSCRE0_EL1);
> +}