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

Halil Pasic
2021-02-10T15:33:43Z

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.