linux-kernel - Re: [PATCH v10 23/40] arm64/signal: Set up and restore the GCS context for signal handlers

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Message-ID: <d5473c27-167f-46c3-9368-487ed4b657cf@sirena.org.uk>
Date: Wed, 21 Aug 2024 19:03:13 +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,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v10 23/40] arm64/signal: Set up and restore the GCS
 context for signal handlers

On Wed, Aug 21, 2024 at 06:28:49PM +0100, Catalin Marinas wrote:
> On Thu, Aug 01, 2024 at 01:06:50PM +0100, Mark Brown wrote:

> > +	ret = copy_from_user(&cap, (__user void*)gcspr_el0, sizeof(cap));
> > +	if (ret)
> > +		return -EFAULT;

> Can the user change GCSPR_EL0 to a non-shadow-stack region, fake the
> cap before sigreturn? copy_from_user() cannot check it's a GCS page.
> Does it actually matter?

We don't take any steps to prevent that since I'm not clear that it
matters, as soon as userspace tries to use the non-GCS page as a GCS it
will fault.  Given the abundance of ways in which a signal handler can
cause a crash it didn't seem worth specific code, the cap token check is
about protecting an actual GCS.

> > +	/*
> > +	 * Push a cap and the GCS entry for the trampoline onto the GCS.
> > +	 */
> > +	put_user_gcs((unsigned long)sigtramp, gcspr_el0 - 2, &ret);
> > +	put_user_gcs(GCS_SIGNAL_CAP(gcspr_el0 - 1), gcspr_el0 - 1, &ret);
> > +	if (ret != 0)
> > +		return ret;

> Doesn't the second put_user_gcs() override the previous ret?

No, we only set ret on error - if the first one faults it'll set ret
then the second one will either leave it unchanged or write the same
error code depending on if it fails.  This idiom is used quite a lot in
the signal code.

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