Re: [PATCH v2 02/49] KVM: x86: Explicitly do runtime CPUID updates "after" initial setup

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> >  	/*
> >  	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> >  	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> > @@ -440,6 +440,15 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
> >  	 * whether the supplied CPUID data is equal to what's already set.
> >  	 */
> >  	if (kvm_vcpu_has_run(vcpu)) {
> > +		/*
> > +		 * Note, runtime CPUID updates may consume other CPUID-driven
> > +		 * vCPU state, e.g. KVM or Xen CPUID bases.  Updating runtime
> > +		 * state before full CPUID processing is functionally correct
> > +		 * only because any change in CPUID is disallowed, i.e. using
> > +		 * stale data is ok because KVM will reject the change.
> > +		 */
> 
> If I understand correctly the sole reason for the below
> __kvm_update_cpuid_runtime is to ensure that kvm_cpuid_check_equal doesn't
> fail because current cpuid also was post-processed with runtime updates.

Yep.

> Can we have a comment stating this? Or even better how about moving the
> call to __kvm_update_cpuid_runtime into the kvm_cpuid_check_equal,
> to emphasize this?

Ya, I'll do both.

> > +		__kvm_update_cpuid_runtime(vcpu, e2, nent);
> > +
> >  		r = kvm_cpuid_check_equal(vcpu, e2, nent);
> >  		if (r)
> >  			return r;
> 
> 
> 
> Overall I am not 100% sure what is better:
> 
> Before the patch it was roughly like this:
> 
> 1. Post process the user given cpuid with bits of KVM runtime state (like xcr0)
> At that point the vcpu->arch.cpuid_entries is stale but consistent, it is just old CPUID.
> 
> 2. kvm_hv_vcpu_init call (IMHO this call can be moved to kvm_vcpu_after_set_cpuid)
> 
> 3. kvm_check_cpuid on the user provided cpuid
> 
> 4. Update the vcpu->arch.cpuid_entries with new and post processed cpuid
> 
> 5. kvm_get_hypervisor_cpuid - I think this also can be cosmetically moved to kvm_vcpu_after_set_cpuid
> 
> 6. kvm_vcpu_after_set_cpuid itself.
> 
> 
> After this change it works like that:
> 
> 1. kvm_hv_vcpu_init (again this belongs more to kvm_vcpu_after_set_cpuid)
> 2. kvm_check_cpuid on the user cpuid without post processing - in theory this can cause bugs
> 3. Update the vcpu->arch.cpuid_entries with new cpuid but without post-processing
> 4. kvm_get_hypervisor_cpuid
> 5. kvm_update_cpuid_runtime
> 6. The old kvm_vcpu_after_set_cpuid
> 
> I'm honestly not sure what is better but IMHO moving the kvm_hv_vcpu_init and
> kvm_get_hypervisor_cpuid into kvm_vcpu_after_set_cpuid would clean up this
> mess a bit regardless of this patch.

It takes many more patches, but doing the swap() allows for the removal of several
APIs that poke into a "raw" kvm_cpuid_entry2 array, and by the end of the series
(with your above feedback addressed) the code gets to (sans comments):

	swap(vcpu->arch.cpuid_entries, e2);
	swap(vcpu->arch.cpuid_nent, nent);

	memcpy(vcpu_caps, vcpu->arch.cpu_caps, sizeof(vcpu_caps));
	BUILD_BUG_ON(sizeof(vcpu_caps) != sizeof(vcpu->arch.cpu_caps));

	if (kvm_vcpu_has_run(vcpu)) {
		r = kvm_cpuid_check_equal(vcpu, e2, nent);
		if (r)
			goto err;
		goto success;
	}

Those are really just bonuses though, the main goal is to prevent recurrences of
bugs where KVM consumes stale vCPU state[*], which is what prompted this change.

[*] https://lore.kernel.org/all/20240228101837.93642-1-vkuznets@redhat.com