Re: [PATCH 1/6] KVM: Explicitly verify target vCPU is online in kvm_get_vcpu()

[Gupta, Pankaj]

On 10/10/2024 5:54 PM, Sean Christopherson wrote:
> On Thu, Oct 10, 2024, Pankaj Gupta wrote:
> > On 10/9/2024 5:04 PM, Sean Christopherson wrote:
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index db567d26f7b9..450dd0444a92 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -969,6 +969,15 @@ static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
> > >    static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
> > >    {
> > >    	int num_vcpus = atomic_read(&kvm->online_vcpus);
> > > +
> > > +	/*
> > > +	 * Explicitly verify the target vCPU is online, as the anti-speculation
> > > +	 * logic only limits the CPU's ability to speculate, e.g. given a "bad"
> > > +	 * index, clamping the index to 0 would return vCPU0, not NULL.
> > > +	 */
> > > +	if (i >= num_vcpus)
> > > +		return NULL;
> >
> > Would sev.c needs a NULL check for?
> >
> > sev_migrate_from()
> > ...
> > src_vcpu = kvm_get_vcpu(src_kvm, i);
> > src_svm = to_svm(src_vcpu);
> > ...
>
> Nope, sev_check_source_vcpus() verifies the source and destination have the same
> number of online vCPUs before calling sev_migrate_from(), and it's all done with
> both VMs locked.
>
> static int sev_check_source_vcpus(struct kvm *dst, struct kvm *src)
> {
> 	struct kvm_vcpu *src_vcpu;
> 	unsigned long i;
>
> 	if (!sev_es_guest(src))
> 		return 0;
>
> 	if (atomic_read(&src->online_vcpus) != atomic_read(&dst->online_vcpus))
> 		return -EINVAL;
>
> 	kvm_for_each_vcpu(i, src_vcpu, src) {
> 		if (!src_vcpu->arch.guest_state_protected)
> 			return -EINVAL;
> 	}
>
> 	return 0;
> }

Yes, right!

Thanks,
Pankaj