Re: [PATCH v12 19/39] arm64/mm: Handle GCS data aborts

From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: 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>,
	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>,
	Kees Cook <kees@kernel.org>, "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>,
	Thiago Jung Bauermann <thiago.bauermann@linaro.org>,
	Ross Burton <ross.burton@arm.com>,
	Yury Khrustalev <yury.khrustalev@arm.com>,
	Wilco Dijkstra <wilco.dijkstra@arm.com>,
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
	kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org,
	linux-arch@vger.kernel.org, linux-mm@kvack.org,
	linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-riscv@lists.infradead.org
Subject: Re: [PATCH v12 19/39] arm64/mm: Handle GCS data aborts
Date: Mon, 2 Sep 2024 19:54:25 +0100	[thread overview]
Message-ID: <ZtYJ4SUDLxrjDDBX@arm.com> (raw)
In-Reply-To: <20240829-arm64-gcs-v12-19-42fec947436a@kernel.org>

On Thu, Aug 29, 2024 at 12:27:35AM +0100, Mark Brown wrote:
> All GCS operations at EL0 must happen on a page which is marked as
> having UnprivGCS access, including read operations.  If a GCS operation
> attempts to access a page without this then it will generate a data
> abort with the GCS bit set in ESR_EL1.ISS2.
> 
> EL0 may validly generate such faults, for example due to copy on write
> which will cause the GCS data to be stored in a read only page with no
> GCS permissions until the actual copy happens.  Since UnprivGCS allows
> both reads and writes to the GCS (though only through GCS operations) we
> need to ensure that the memory management subsystem handles GCS accesses
> as writes at all times.  Do this by adding FAULT_FLAG_WRITE to any GCS
> page faults, adding handling to ensure that invalid cases are identfied
> as such early so the memory management core does not think they will
> succeed.  The core cannot distinguish between VMAs which are generally
> writeable and VMAs which are only writeable through GCS operations.
> 
> EL1 may validly write to EL0 GCS for management purposes (eg, while
> initialising with cap tokens).
> 
> We also report any GCS faults in VMAs not marked as part of a GCS as
> access violations, causing a fault to be delivered to userspace if it
> attempts to do GCS operations outside a GCS.
> 
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> Signed-off-by: Mark Brown <broonie@kernel.org>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>