linux-kernel - Re: [PATCH v13 16/40] KVM: arm64: Manage GCS access and registers for guests

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Message-ID: <1ec14d13-7db6-4642-a14b-db8ae362e594@sirena.org.uk>
Date: Thu, 3 Oct 2024 15:50:47 +0100
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.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,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v13 16/40] KVM: arm64: Manage GCS access and registers
 for guests

On Wed, Oct 02, 2024 at 08:29:28PM +0100, Marc Zyngier wrote:
> Mark Brown <broonie@...nel.org> wrote:

> > They are, though really they should UNDEF if GCS isn't there (which I
> > had thought was what you were referencing here).  Equally we only have
> > traps for a subset of GCS instructions and it's not like there aren't a
> > whole bunch of untrappable extensions anyway so it's not clear it's
> > worth the effort just for that.

> If the encodings UNDEF when GCS is not implemented (i.e. they are not
> in the NOP space), then all trapable instructions should absolutely
> UNDEF (and yes, it is worth the effort, even if it is only to
> demonstrate that the architecture is sub-par).

Yes, see DDI0487 K.a C5.9.  If you're concerned about being unable to
generate UNDEFs there's a rather large set of existing extensions where
that's not possible, most of the hwcaps in the hwcap selftest that don't
set sigill_reliable but do have a SIGILL generator for a start.

> So I expect the next version to handle traps for GCSPUSHX, GCSPOPX,
> GCSPUSHM, GCSSTR and GCSSTTR when GCS isn't enabled.

OK, I already had that change locally after your first message.

> I'm also pretty sure this is missing some form of sanitisation for
> PSTATE.EXLOCK, and looking at the pseudocode, you seem to be missing
> the handling of that bit on exception injection.

Ah, yes - I think I see the missing exception injection handling in
enter_exception64().  I'm not seeing what you're referencing with
sanitisation though, could you give me some more specific pointers
please?

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