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

[Tony Krowiak]

On 2/10/21 5:46 PM, Halil Pasic wrote:
> On Wed, 10 Feb 2021 17:05:48 -0500
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > On 2/10/21 10:32 AM, Halil Pasic wrote:
> > > On Wed, 10 Feb 2021 16:24:29 +0100
> > > Halil Pasic <pasic@xxxxxxxxxxxxx> 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?

Yes, it makes sense. I guess I didn't look closely at your
suggestion when I said it was exactly what I implemented
after agreeing with Connie. I had a slight difference in
my implementation:

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;
        mutex_unlock(&matrix_dev->lock);
        kvm_arch_crypto_clear_masks(kvm);
        mutex_lock(&matrix_dev->lock);
        kvm->arch.crypto.pqap_hook = NULL;
        vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
        matrix_mdev->kvm = NULL;
        kvm_put_kvm(kvm);
    }

    mutex_unlock(&matrix_dev->lock);
}

In your scenario, the unassignment would fail with -EBUSY because
the matrix_mdev->kvm pointer would not have yet been
cleared. The other problem with your implementation is that
IRQ resources would not get cleared after the reset because
the matrix_mdev->kvm pointer would be NULL at that time.