linux-kernel - Re: [PATCH v10 05/40] arm64/gcs: Document the ABI for Guarded Control Stacks

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Message-ID: <7c17b28e-a5d1-4113-9580-9501692af513@sirena.org.uk>
Date: Fri, 16 Aug 2024 13:02:22 +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,
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	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v10 05/40] arm64/gcs: Document the ABI for Guarded
 Control Stacks

On Fri, Aug 16, 2024 at 12:09:01PM +0100, Catalin Marinas wrote:
> On Thu, Aug 01, 2024 at 01:06:32PM +0100, Mark Brown wrote:

> > +* EL0 GCS entries with bit 63 set are reserved for use, one such use is defined

> Maybe "reserved for specific uses". The proposed sentenced feels like
> it's missing something.

Actually we removed the usage of bit 63 so I'll just drop this.

> > +* When a new thread is created by a thread which has GCS enabled then a
> > +  new Guarded Control Stack will be allocated for the new thread with
> > +  half the size of the standard stack.

> Is the half size still the case? It also seems a bit inconsistent to
> have RLIMIT_STACK when GCS is enabled and half the stack size when a new
> thread is created.

Yes, this predates the rebase onto clone3() - I'll update.

> [...]
> > +* When a thread is freed the Guarded Control Stack initially allocated for
> > +  that thread will be freed.  Note carefully that if the stack has been
> > +  switched this may not be the stack currently in use by the thread.

> Is this true for shadow stacks explicitly allocated by the user with
> map_shadow_stack()?

It is only true for the stacks allocaeted by the kernel, if we didn't
allocate a stack we don't free it.

> > +* The signal handler will use the same GCS as the interrupted context.

> I assume this is true even with sigaltstack. Not easy to have
> alternative shadow stack without additional ABI.

Yes.

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