linux-kernel - Re: [PATCH v7 24/39] arm64/signal: Set up and restore the GCS context for signal handlers

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Message-ID: <ZXRnGbYI+uq7m9sA@finisterre.sirena.org.uk>
Date:   Sat, 9 Dec 2023 13:09:45 +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 24/39] arm64/signal: Set up and restore the GCS
 context for signal handlers

On Sat, Dec 09, 2023 at 12:15:22AM -0300, Thiago Jung Bauermann wrote:
> Mark Brown <broonie@...nel.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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