[PATCH] KVM: arm64: Prevent mixed-width VM creation

[Mark Rutland]

On Thu, May 20, 2021 at 01:58:55PM +0100, Marc Zyngier wrote:
> On Thu, 20 May 2021 13:44:34 +0100,
> Mark Rutland <mark.rutland at arm.com> wrote:
> > 
> > On Thu, May 20, 2021 at 01:22:53PM +0100, Marc Zyngier wrote:
> > > It looks like we have tolerated creating mixed-width VMs since...
> > > forever. However, that was never the intention, and we'd rather
> > > not have to support that pointless complexity.
> > > 
> > > Forbid such a setup by making sure all the vcpus have the same
> > > register width.
> > > 
> > > Reported-by: Steven Price <steven.price at arm.com>
> > > Signed-off-by: Marc Zyngier <maz at kernel.org>
> > > Cc: stable at vger.kernel.org
> > > ---
> > >  arch/arm64/kvm/reset.c | 28 ++++++++++++++++++++++++----
> > >  1 file changed, 24 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > > index 956cdc240148..1cf308be6ef3 100644
> > > --- a/arch/arm64/kvm/reset.c
> > > +++ b/arch/arm64/kvm/reset.c
> > > @@ -166,6 +166,25 @@ static int kvm_vcpu_enable_ptrauth(struct kvm_vcpu *vcpu)
> > >  	return 0;
> > >  }
> > >  
> > > +static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
> > > +{
> > > +	struct kvm_vcpu *tmp;
> > > +	int i;
> > > +
> > > +	/* Check that the vcpus are either all 32bit or all 64bit */
> > > +	kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
> > > +		bool w;
> > > +
> > > +		w  = test_bit(KVM_ARM_VCPU_EL1_32BIT, tmp->arch.features);
> > > +		w ^= test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features);
> > > +
> > > +		if (w)
> > > +			return false;
> > > +	}
> > 
> > I think this is wrong for a single-cpu VM. In that case, the loop will
> > have a single iteration, and tmp == vcpu, so w must be 0 regardless of
> > the value of arch.features.
> 
> I don't immediately see what is wrong with a single-cpu VM. 'w' will
> be zero indeed, and we'll return that this is allowed. After all, each
> VM starts by being a single-CPU VM.

Sorry; I should have been clearer. I had assumed that this was trying to
rely on a difference across vcpus implicitly providing an equivalent of
the removed check for the KVM_ARM_VCPU_EL1_32BIT cap. I guess from the
below that was not the case. :)

Thanks,
Mark.

> But of course...
> 
> > IIUC that doesn't prevent KVM_ARM_VCPU_EL1_32BIT being set when we don't
> > have the ARM64_HAS_32BIT_EL1 cap, unless that's checked elsewhere?
> 
> ... I mistakenly removed the check against ARM64_HAS_32BIT_EL1...
> 
> > 
> > How about something like:
> > 
> > | static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
> > | {
> > | 	bool is_32bit = vcpu_features_32bit(vcpu);
> > | 	struct kvm_vcpu *tmp;
> > | 	int i;
> > | 
> > | 	if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) && is_32bit)
> > | 		return false;
> > | 
> > | 	kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
> > | 		if (is_32bit != vcpu_features_32bit(tmp))
> > | 			return false;
> > | 	}
> > | 
> > | 	return true;
> > | }
> > 
> > ... with a helper in <asm/kvm_emulate.h> like:
> > 
> > | static bool vcpu_features_32bit(struct kvm_vcpu *vcpu)
> > | {
> > | 	return test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features);
> > | }
> > 
> > ... or
> > 
> > | static inline bool vcpu_has_feature(struct kvm_vcpu *vcpu, int feature)
> > | {
> > | 	return test_bit(feature, vcpu->arch.features);
> > | }
> > 
> > ... so that we can avoid the line splitting required by the length of
> > the test_bit() expression?
> 
> Yup, looks OK to me (with a preference for the latter).
> 
> Thanks,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.