Re: [PATCH v3 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks

[Tony Krowiak]

On 3/3/21 2:47 PM, Halil Pasic wrote:
> On Wed, 3 Mar 2021 12:10:11 -0500
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > On 3/3/21 10:23 AM, Halil Pasic wrote:
> > > On Tue,  2 Mar 2021 15:43:22 -0500
> > > Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
> > >   
> > > > This patch fixes a lockdep splat introduced by commit f21916ec4826
> > > > ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated").
> > > > The lockdep splat only occurs when starting a Secure Execution guest.
> > > > Crypto virtualization (vfio_ap) is not yet supported for SE guests;
> > > > however, in order to avoid this problem when support becomes available,
> > > > this fix is being provided.
> > > [..]
> > >   
> > > > @@ -1038,14 +1116,28 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
> > > >    {
> > > >    	struct ap_matrix_mdev *m;
> > > >
> > > > -	list_for_each_entry(m, &matrix_dev->mdev_list, node) {
> > > > -		if ((m != matrix_mdev) && (m->kvm == kvm))
> > > > -			return -EPERM;
> > > > -	}
> > > > +	if (kvm->arch.crypto.crycbd) {
> > > > +		matrix_mdev->kvm_busy = true;
> > > >
> > > > -	matrix_mdev->kvm = kvm;
> > > > -	kvm_get_kvm(kvm);
> > > > -	kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
> > > > +		list_for_each_entry(m, &matrix_dev->mdev_list, node) {
> > > > +			if ((m != matrix_mdev) && (m->kvm == kvm)) {
> > > > +				wake_up_all(&matrix_mdev->wait_for_kvm);
> > > This ain't no good. kvm_busy will remain true if we take this exit. The
> > > wake_up_all() is not needed, because we hold the lock, so nobody can
> > > observe it if we don't forget kvm_busy set.
> > >
> > > I suggest moving matrix_mdev->kvm_busy = true; after this loop, maybe right
> > > before the unlock, and removing the wake_up_all().
> > >   
> > > > +				return -EPERM;
> > > > +			}
> > > > +		}
> > > > +
> > > > +		kvm_get_kvm(kvm);
> > > > +		mutex_unlock(&matrix_dev->lock);
> > > > +		kvm_arch_crypto_set_masks(kvm,
> > > > +					  matrix_mdev->matrix.apm,
> > > > +					  matrix_mdev->matrix.aqm,
> > > > +					  matrix_mdev->matrix.adm);
> > > > +		mutex_lock(&matrix_dev->lock);
> > > > +		kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
> > > > +		matrix_mdev->kvm = kvm;
> > > > +		matrix_mdev->kvm_busy = false;
> > > > +		wake_up_all(&matrix_mdev->wait_for_kvm);
> > > > +	}
> > > >
> > > >    	return 0;
> > > >    }
> > > [..]
> > >   
> > > > @@ -1300,7 +1406,21 @@ static ssize_t vfio_ap_mdev_ioctl(struct mdev_device *mdev,
> > > >    		ret = vfio_ap_mdev_get_device_info(arg);
> > > >    		break;
> > > >    	case VFIO_DEVICE_RESET:
> > > > -		ret = vfio_ap_mdev_reset_queues(mdev);
> > > > +		matrix_mdev = mdev_get_drvdata(mdev);
> > > > +
> > > > +		/*
> > > > +		 * If the KVM pointer is in the process of being set, wait until
> > > > +		 * the process has completed.
> > > > +		 */
> > > > +		wait_event_cmd(matrix_mdev->wait_for_kvm,
> > > > +			       matrix_mdev->kvm_busy == false,
> > > > +			       mutex_unlock(&matrix_dev->lock),
> > > > +			       mutex_lock(&matrix_dev->lock));
> > > > +
> > > > +		if (matrix_mdev->kvm)
> > > > +			ret = vfio_ap_mdev_reset_queues(mdev);
> > > > +		else
> > > > +			ret = -ENODEV;
> > > I don't think rejecting the reset is a good idea. I have you a more detailed
> > > explanation of the list, where we initially discussed this question.
> > >
> > > How do you exect userspace to react to this -ENODEV?
> > After reading your more detailed explanation, I have come to the
> > conclusion that the test for matrix_mdev->kvm should not be
> > performed here and the the vfio_ap_mdev_reset_queues() function
> > should be called regardless. Each queue assigned to the mdev
> > that is also bound to the vfio_ap driver will get reset and its
> > IRQ resources cleaned up if they haven't already been and the
> > other required conditions are met (i.e., see
> > vfio_ap_mdev_free_irq_resources()).
> My point is if !->kvm the other required conditions are not met. But
> yes we can go back to unconditional vfio_ap_mdev_reset_queues(mdev),
> and think about the necessity of performing a
> vfio_ap_mdev_reset_queues() if !->kvm later as I proposed in the other
> mail.

The other conditions may or may not have been met depending
upon whether ->kvm is NULL because the VFIO_DEVICE_RESET
ioctl was invoked while the matrix_dev->lock was released
in the vfio_ap_mdev_unset_kvm() function. If that was the case,
then there is no need to clean up the IRQ resources because it
will already have been done.

On the other hand, if we don't have ->kvm because something broke,
then we may be out of luck anyway. There will certainly be no
way to unregister the GISC; however, it may still be possible
to unpin the pages if we still have q->saved_pfn.

The point is, if the queue is bound to vfio_ap, it can be reset. If we can't
clean up the IRQ resources because something is broken, then there
is nothing we can do about that.


> Regards,
> Halil