Re: [PATCH 0/3] KVM: x86: guest interface for SEV live migration

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 20, 2021, Sean Christopherson wrote:
> On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> > On 20/04/21 19:31, Sean Christopherson wrote:
> > > > +	case KVM_HC_PAGE_ENC_STATUS: {
> > > > +		u64 gpa = a0, npages = a1, enc = a2;
> > > > +
> > > > +		ret = -KVM_ENOSYS;
> > > > +		if (!vcpu->kvm->arch.hypercall_exit_enabled)
> > > 
> > > I don't follow, why does the hypercall need to be gated by a capability?  What
> > > would break if this were changed to?
> > > 
> > > 		if (!guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS))
> > 
> > The problem is that it's valid to take KVM_GET_SUPPORTED_CPUID and send it
> > unmodified to KVM_SET_CPUID2.  For this reason, features that are
> > conditional on other ioctls, or that require some kind of userspace support,
> > must not be in KVM_GET_SUPPORTED_CPUID.  For example:
> > 
> > - TSC_DEADLINE because it is only implemented after KVM_CREATE_IRQCHIP (or
> > after KVM_ENABLE_CAP of KVM_CAP_IRQCHIP_SPLIT)
> > 
> > - MONITOR only makes sense if userspace enables KVM_CAP_X86_DISABLE_EXITS
> > 
> > X2APIC is reported even though it shouldn't be.  Too late to fix that, I
> > think.
> > 
> > In this particular case, if userspace sets the bit in CPUID2 but doesn't
> > handle KVM_EXIT_HYPERCALL, the guest will probably trigger some kind of
> > assertion failure as soon as it invokes the HC_PAGE_ENC_STATUS hypercall.
> 
> Gah, I was thinking of the MSR behavior and forgot that the hypercall exiting
> behavior intentionally doesn't require extra filtering.
> 
> It's also worth noting that guest_pv_has() is particularly useless since it
> will unconditionally return true for older VMMs that dont' enable
> KVM_CAP_ENFORCE_PV_FEATURE_CPUID.
> 
> Bummer.

Oh!  Almost forgot my hail mary idea.  Instead of a new capability, can we
reject the hypercall if userspace has _not_ set KVM_CAP_ENFORCE_PV_FEATURE_CPUID?

			if (vcpu->arch.pv_cpuid.enforce &&
			    !guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS)
				break;