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Date: Tue, 11 Feb 2025 13:47:28 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Joey Gouly <joey.gouly@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Zenghui Yu <yuzenghui@huawei.com>, Fuad Tabba <tabba@google.com>
Subject: Re: [PATCH 06/18] KVM: arm64: Plug FEAT_GCS handling
Message-ID: <Z6tU8IyAr8lp9IYI@J2N7QTR9R3>
References: <20250210184150.2145093-1-maz@kernel.org>
 <20250210184150.2145093-7-maz@kernel.org>
 <Z6tEUzwcHVHALIdu@J2N7QTR9R3>
 <861pw4txhh.wl-maz@kernel.org>
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In-Reply-To: <861pw4txhh.wl-maz@kernel.org>

On Tue, Feb 11, 2025 at 01:35:54PM +0000, Marc Zyngier wrote:
> On Tue, 11 Feb 2025 12:36:35 +0000,
> Mark Rutland <mark.rutland@arm.com> wrote:
> > On Mon, Feb 10, 2025 at 06:41:37PM +0000, Marc Zyngier wrote:

> > > +static int kvm_handle_gcs(struct kvm_vcpu *vcpu)
> > > +{
> > > +	/* We don't expect GCS, so treat it with contempt */
> > > +	if (kvm_has_feat(vcpu->kvm, ID_AA64PFR1_EL1, GCS, IMP))
> > > +		WARN_ON_ONCE(1);
> > 
> > Just to check / better my understanging, do we enforce that this can't
> > be exposed to the guest somewhere?
> > 
> > I see __kvm_read_sanitised_id_reg() masks it out, and the sys_reg_descs
> > table has it filtered, but I'm not immediately sure whether that
> > prevents host userspace maliciously setting this?
> 
> On writing to the idreg, you end-up in set_id_aa64pfr1_el1(), which
> calls into set_id_reg(). There, arm64_check_features() compares each
> and every feature in that register with the mask and limits that have
> been established.
> 
> Since GCS is not part of the writable mask, and that it has been
> disabled, the only valid value for ID_AA64PFR1_EL1.GCS is 0. A
> non-zero value provided by userspace will be caught by the last check
> in arm64_check_features(), and an error be returned.

Perfect, thanks!

Mark.