Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context

[Tom Lendacky]

On 2/24/21 9:44 PM, Steve Rutherford wrote:
> On Wed, Feb 24, 2021 at 1:00 AM Nathan Tempelman <natet@xxxxxxxxxx> wrote:
> > @@ -1186,6 +1195,10 @@ int svm_register_enc_region(struct kvm *kvm,
> >          if (!sev_guest(kvm))
> >                  return -ENOTTY;
> >
> > +       /* If kvm is mirroring encryption context it isn't responsible for it */
> > +       if (is_mirroring_enc_context(kvm))
> > +               return -ENOTTY;
> > +
>
> Is this necessary? Same for unregister. When we looked at
> sev_pin_memory, I believe we concluded that double pinning was safe.
> >          if (range->addr > ULONG_MAX || range->size > ULONG_MAX)
> >                  return -EINVAL;
> >
> > @@ -1252,6 +1265,10 @@ int svm_unregister_enc_region(struct kvm *kvm,
> >          struct enc_region *region;
> >          int ret;
> >
> > +       /* If kvm is mirroring encryption context it isn't responsible for it */
> > +       if (is_mirroring_enc_context(kvm))
> > +               return -ENOTTY;
> > +
> >          mutex_lock(&kvm->lock);
> >
> >          if (!sev_guest(kvm)) {
> > @@ -1282,6 +1299,65 @@ int svm_unregister_enc_region(struct kvm *kvm,
> >          return ret;
> >   }
> >
> > +int svm_vm_copy_asid_to(struct kvm *kvm, unsigned int mirror_kvm_fd)
> > +{
> > +       struct file *mirror_kvm_file;
> > +       struct kvm *mirror_kvm;
> > +       struct kvm_sev_info *mirror_kvm_sev;
> > +       unsigned int asid;
> > +       int ret;
> > +
> > +       if (!sev_guest(kvm))
> > +               return -ENOTTY;
>
> You definitely don't want this: this is the function that turns the vm
> into an SEV guest (marks SEV as active).

The sev_guest() function does not set sev->active, it only checks it. The 
sev_guest_init() function is where sev->active is set.

> (Not an issue with this patch, but a broader issue) I believe
> sev_guest lacks the necessary acquire/release barriers on sev->active,

The svm_mem_enc_op() takes the kvm lock and that is the only way into the 
sev_guest_init() function where sev->active is set.

Thanks,
Tom

> since it's called without the kvm lock. I mean, it's x86, so the only
> one that's going to hose you is the compiler for this type of access.
> There should be an smp_rmb() after the access in sev_guest and an
> smp_wmb() before the access in SEV_GUEST_INIT and here.
> > +
> > +       mutex_lock(&kvm->lock);
> > +
> > +       /* Mirrors of mirrors should work, but let's not get silly */
> > +       if (is_mirroring_enc_context(kvm)) {
> > +               ret = -ENOTTY;
> > +               goto failed;
> > +       }
> > +
> > +       mirror_kvm_file = fget(mirror_kvm_fd);
> > +       if (!kvm_is_kvm(mirror_kvm_file)) {
> > +               ret = -EBADF;
> > +               goto failed;
> > +       }
> > +
> > +       mirror_kvm = mirror_kvm_file->private_data;
> > +
> > +       if (mirror_kvm == kvm || is_mirroring_enc_context(mirror_kvm)) {
> Just check if the source is an sev_guest and that the destination is
> not an sev_guest.
>
> I reviewed earlier incarnations of this, and think the high-level idea
> is sound. I'd like to see kvm-selftests for this patch, and plan on
> collaborating with AMD to help make those happen.