linux-kernel - Re: [PATCH v10 19/40] arm64/gcs: Context switch GCS state for EL0

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Message-ID: <3bc63a7e-0cb7-483b-9054-b52727997b6d@sirena.org.uk>
Date: Wed, 21 Aug 2024 13:48:22 +0100
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
Cc: 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>,
	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>, Kees Cook <kees@...nel.org>,
	"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>,
	Ross Burton <ross.burton@....com>,
	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 v10 19/40] arm64/gcs: Context switch GCS state for EL0

On Wed, Aug 21, 2024 at 09:50:22AM +0100, Catalin Marinas wrote:
> On Tue, Aug 20, 2024 at 06:56:19PM +0100, Mark Brown wrote:

> > I forgot when writing the above that we always allow reads from
> > GCSPR_EL0 in order to avoid corner cases for unwinders in the case of
> > asynchronous disable.  I'd expect that to be cheap to access though.

> But then gcs_preserve_current_state() doesn't save the GCSPR_EL0 value
> if the shadow stack was disabled. At the subsequent switch to this task,
> we write some stale value.

True, we should make the disable save the current value.

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