linux-kernel - Re: [PATCH v11 24/39] arm64/signal: Set up and restore the GCS context for signal handlers

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Message-ID: <ZshSUcVIsdeJKcc5@arm.com>
Date: Fri, 23 Aug 2024 10:11:45 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Brown <broonie@...nel.org>
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>,
	Yury Khrustalev <yury.khrustalev@....com>,
	Wilco Dijkstra <wilco.dijkstra@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
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Subject: Re: [PATCH v11 24/39] arm64/signal: Set up and restore the GCS
 context for signal handlers

On Thu, Aug 22, 2024 at 02:15:27AM +0100, Mark Brown wrote:
> When invoking a signal handler we use the GCS configuration and stack
> for the current thread.
> 
> Since we implement signal return by calling the signal handler with a
> return address set up pointing to a trampoline in the vDSO we need to
> also configure any active GCS for this by pushing a frame for the
> trampoline onto the GCS.  If we do not do this then signal return will
> generate a GCS protection fault.
> 
> In order to guard against attempts to bypass GCS protections via signal
> return we only allow returning with GCSPR_EL0 pointing to an address
> where it was previously preempted by a signal.  We do this by pushing a
> cap onto the GCS, this takes the form of an architectural GCS cap token
> with the top bit set and token type of 0 which we add on signal entry
> and validate and pop off on signal return.  The combination of the top
> bit being set and the token type mean that this can't be interpreted as
> a valid token or address.
> 
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@...aro.org>
> Signed-off-by: Mark Brown <broonie@...nel.org>

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