linux-kernel - Re: [PATCH v10 24/40] arm64/signal: Expose GCS state in signal frames

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Message-ID: <280e4455-9cf7-40e1-9114-7bb3aa9de868@sirena.org.uk>
Date: Thu, 15 Aug 2024 16:46:04 +0100
From: Mark Brown <broonie@...nel.org>
To: Dave Martin <Dave.Martin@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	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 24/40] arm64/signal: Expose GCS state in signal frames

On Thu, Aug 15, 2024 at 04:33:25PM +0100, Dave Martin wrote:
> On Thu, Aug 15, 2024 at 04:05:32PM +0100, Mark Brown wrote:

> > The expectation (at least for arm64) is that the main program will only
> > have shadow stacks if everything says it can support them.  If the
> > dynamic linker turns them on during startup prior to parsing the main
> > executables this means that it should turn them off before actually
> > starting the executable, taking care to consider any locking of features.

> Hmm, so we really do get a clear "enable shadow stack" call to the
> kernel, which we can reasonaly expect won't happen for ancient software?

Yes, userspace always has to explicitly enable the GCS.

> If so, I think dumping the GCS state in the sigframe could be made
> conditional on that without problems (?)

It is - we only allocate the sigframe if the task has GCS enabled.

> > > Related question: does shadow stack work with ucontext-based coroutines?
> > > Per-context stacks need to be allocated by the program for that.

> > Yes, ucontext based coroutines are the sort of thing I meant when I was
> > talking about returning to a different context?  

> Ah, right.  Doing this asynchronously on the back of a signal (instead
> of doing a sigreturn) is the bad thing.  setcontext() officially
> doesn't work for this any more, and doing it by hacking or rebuilding
> the sigframe is extremely hairy and probably a terrible idea for the
> reasons I gave.

I see.  I tend to view this as more adventurous than I personally would
be when writing userspace code but equally I don't see a need to
actively break things.  There's no *requirement* to use libc...

> So, overall I think making ucontext coroutines with with GCS is purely
> a libc matter that is "interesting" here, but we don't need to worry
> about.

Yes, it's not our problem so long as we don't get in the way somehow.

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