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

Halil Pasic
2021-02-10T22:47:10Z

On Wed, 10 Feb 2021 17:05:48 -0500
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

> On 2/10/21 10:32 AM, Halil Pasic wrote:
> > On Wed, 10 Feb 2021 16:24:29 +0100
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> >
> >>> Maybe you could
> >>> - grab a reference to kvm while holding the lock
> >>> - call the mask handling functions with that kvm reference
> >>> - lock again, drop the reference, and do the rest of the processing?
> >> I agree, matrix_mdev->kvm can go NULL any time and we are risking
> >> a null pointer dereference here.
> >>
> >> Another idea would be to do
> >>
> >>
> >> static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
> >> {
> >> struct kvm *kvm;
> >>
> >> mutex_lock(&matrix_dev->lock);
> >> if (matrix_mdev->kvm) {
> >> kvm = matrix_mdev->kvm;
> >> matrix_mdev->kvm = NULL;
> >> mutex_unlock(&matrix_dev->lock);
> >> kvm_arch_crypto_clear_masks(kvm);
> >> mutex_lock(&matrix_dev->lock);
> >> matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> > s/matrix_mdev->kvm/kvm
> >> vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> >> kvm_put_kvm(kvm);
> >> }
> >> mutex_unlock(&matrix_dev->lock);
> >> }
> >>
> >> That way only one unset would actually do the unset and cleanup
> >> and every other invocation would bail out with only checking
> >> matrix_mdev->kvm.
> > But the problem with that is that we enable the the assign/unassign
> > prematurely, which could interfere wit reset_queues(). Forget about
> > it.
>
> Not sure what you mean by this.
>
>

I mean because above I first do
(1) matrix_mdev->kvm = NULL;
and then do
(2) vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
another thread could do
static ssize_t unassign_adapter_store(struct device *dev,
struct device_attribute *attr,
const char *buf, size_t count)
{
int ret;
unsigned long apid;
struct mdev_device *mdev = mdev_from_dev(dev);
struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);

/* If the guest is running, disallow un-assignment of adapter */
if (matrix_mdev->kvm)
return -EBUSY;
...
}
between (1) and (2), and we would not bail out with -EBUSY because !!kvm
because of (1). That means we would change matrix_mdev->matrix and we
would not reset the queues that correspond to the apid that was just
removed, because by the time we do the reset_queues, the queues are
not in the matrix_mdev->matrix any more.

Does that make sense?