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Date:   Sat, 9 Dec 2023 13:09:45 +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>,
        Christian Brauner <brauner@kernel.org>,
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Subject: Re: [PATCH v7 24/39] arm64/signal: Set up and restore the GCS
 context for signal handlers
Message-ID: <ZXRnGbYI+uq7m9sA@finisterre.sirena.org.uk>
References: <20231122-arm64-gcs-v7-0-201c483bd775@kernel.org>
 <20231122-arm64-gcs-v7-24-201c483bd775@kernel.org>
 <8734wcgj79.fsf@linaro.org>
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On Sat, Dec 09, 2023 at 12:15:22AM -0300, Thiago Jung Bauermann wrote:
> Mark Brown <broonie@kernel.org> writes:

> > +	/* The cap must have the low bits set to a token value */
> > +	if (GCS_CAP_TOKEN(val) != 0)
> > +		return false;

> I found the comment above a little confusing, since the if condition
> actually checks that low bits aren't set at all. Perhaps reword to
> something like "The token value of a signal cap must be 0"?

Right, that's bitrot from the previous token format.

> I'm still not proficient enough in GCS to know how exactly this could be
> abused (e.g., somehow writing the desired return location right above
> one of these inactive caps and arranging for GCSPR to point to the cap
> before returning from a signal) but to be safe or paranoid, perhaps zero
> the location of the cap before returning?

Right, ideally we'd be doing a compare and exchange here to substitute
in a zero.

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