linux-kernel - Re: [PATCH v13 22/40] arm64/gcs: Ensure that new threads have a GCS

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Message-ID: <Zv_PFVffwIkKevzE@arm.com>
Date: Fri, 4 Oct 2024 14:18:45 +0300
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>,
	David Spickett <david.spickett@....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,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
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	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v13 22/40] arm64/gcs: Ensure that new threads have a GCS

On Tue, Oct 01, 2024 at 11:59:01PM +0100, Mark Brown wrote:
> When a new thread is created by a thread with GCS enabled the GCS needs
> to be specified along with the regular stack.
> 
> Unfortunately plain clone() is not extensible and existing clone3()
> users will not specify a stack so all existing code would be broken if
> we mandated specifying the stack explicitly.  For compatibility with
> these cases and also x86 (which did not initially implement clone3()
> support for shadow stacks) if no GCS is specified we will allocate one
> so when a thread is created which has GCS enabled allocate one for it.
> We follow the extensively discussed x86 implementation and allocate
> min(RLIMIT_STACK/2, 2G).  Since the GCS only stores the call stack and not
> any variables this should be more than sufficient for most applications.
> 
> GCSs allocated via this mechanism will be freed when the thread exits.

I think Szabolcs mentioned a GCS leak with v12:

https://lore.kernel.org/r/ZtrihWQFyb2/XrQV@arm.com

(and in some private messages IIRC)

Has this been identified? The changelog only mentions a leak in v8.

-- 
Catalin