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Date:   Sun, 10 Dec 2023 14:22:56 +0000
From:   Mark Brown <broonie@kernel.org>
To:     Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, Oleg Nesterov <oleg@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Shuah Khan <shuah@kernel.org>,
        "Rick P. Edgecombe" <rick.p.edgecombe@intel.com>,
        Deepak Gupta <debug@rivosinc.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
        "H.J. Lu" <hjl.tools@gmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Florian Weimer <fweimer@redhat.com>,
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Subject: Re: [PATCH v7 26/39] arm64/ptrace: Expose GCS via ptrace and core
 files
Message-ID: <ZXXJwNdKC/y6bRYn@finisterre.sirena.org.uk>
References: <20231122-arm64-gcs-v7-0-201c483bd775@kernel.org>
 <20231122-arm64-gcs-v7-26-201c483bd775@kernel.org>
 <877clney35.fsf@linaro.org>
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On Sat, Dec 09, 2023 at 08:49:02PM -0300, Thiago Jung Bauermann wrote:
> Mark Brown <broonie@kernel.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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