linux-kernel - Re: [PATCH v7 26/39] arm64/ptrace: Expose GCS via ptrace and core files

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Message-ID: <ZXXJwNdKC/y6bRYn@finisterre.sirena.org.uk>
Date:   Sun, 10 Dec 2023 14:22:56 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Thiago Jung Bauermann <thiago.bauermann@...aro.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>,
        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>,
        Florian Weimer <fweimer@...hat.com>,
        Christian Brauner <brauner@...nel.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 26/39] arm64/ptrace: Expose GCS via ptrace and core
 files

On Sat, Dec 09, 2023 at 08:49:02PM -0300, Thiago Jung Bauermann wrote:
> Mark Brown <broonie@...nel.org> writes:

> > Provide a new register type NT_ARM_GCS reporting the current GCS mode
> > and pointer for EL0.  Due to the interactions with allocation and
> > deallocation of Guarded Control Stacks we do not permit any changes to
> > the GCS mode via ptrace, only GCSPR_EL0 may be changed.

> The code allows disabling GCS. Is that unintended?

No, it's intentional - ptrace has a lot of control over the process,
there's not a huge point trying to protect against it doing a disable.
The reason we prevent enabling is the allocation of a GCS along with
enable, the complexity of doing that on a remote process seemed
unjustified.  If clone3() ends up allowing manual allocation and
placement that'll likely be revised.

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