Re: [PATCH] s390/vfio-ap: Clean up vfio_ap resources when KVM pointer invalidated

[Tony Krowiak]

On 12/4/20 2:05 PM, Halil Pasic wrote:
> On Fri, 4 Dec 2020 09:43:59 -0500
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > > > +{
> > > > +	if (matrix_mdev->kvm) {
> > > > +		(matrix_mdev->kvm);
> > > > +		matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> > > Is a plain assignment to arch.crypto.pqap_hook apropriate, or do we need
> > > to take more care?
> > >
> > > For instance kvm_arch_crypto_set_masks() takes kvm->lock before poking
> > > kvm->arch.crypto.crycb.
> > I do not think so. The CRYCB is used by KVM to provide crypto resources
> > to the guest so it makes sense to protect it from changes to it while
> > passing
> > the AP devices through to the guest. The hook is used only when an AQIC
> > executed on the guest is intercepted by KVM. If the notifier
> > is being invoked to notify vfio_ap that KVM has been set to NULL, this means
> > the guest is gone in which case there will be no AP instructions to
> > intercept.
> If the update to pqap_hook isn't observed as atomic we still have a
> problem. With torn writes or reads we would try to use a corrupt function
> pointer. While the compiler probably ain't likely to generate silly code
> for the above assignment (multiple write instructions less then
> quadword wide), I know of nothing that would prohibit the compiler to do
> so.

I see that in the handle_pqap() function in arch/s390/kvm/priv.c
that gets called when the AQIC instruction is intercepted,
the pqap_hook is protected by locking the owner of the hook:

        if (!try_module_get(vcpu->kvm->arch.crypto.pqap_hook->owner))
            return -EOPNOTSUPP;
        ret = vcpu->kvm->arch.crypto.pqap_hook->hook(vcpu);
module_put(vcpu->kvm->arch.crypto.pqap_hook->owner);

Maybe that is what we should do when the kvm->arch.crypto.pqap_hook
is set to NULL?

> I'm not certain about the scope of the kvm->lock (if it's supposed to
> protect the whole sub-tree of objects). Maybe Janosch can help us out.
> @Janosch: what do you think?
>
> Regards,
> Halil