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

[Marc Zyngier <maz@kernel.org>]

On Thu, 20 May 2021 13:44:34 +0100,
Mark Rutland <mark.rutland@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@arm.com>
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > Cc: stable@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.

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.