Re: [PATCH 06/18] KVM: arm64: Plug FEAT_GCS handling

[Mark Rutland <mark.rutland@arm.com>]

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.