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

Sean Christopherson
2024-10-10T15:54:24Z

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;
}