On Wed, 10 Feb 2021 11:53:34 +0100 Cornelia Huck cohuck@redhat.com wrote:
On Tue, 9 Feb 2021 14:48:30 -0500 Tony Krowiak akrowiak@linux.ibm.com wrote:
This patch fixes a circular locking dependency in the CI introduced by commit f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated"). The lockdep only occurs when starting a Secure Execution guest. Crypto virtualization (vfio_ap) is not yet supported for SE guests; however, in order to avoid CI errors, this fix is being provided.
The circular lockdep was introduced when the masks in the guest's APCB were taken under the matrix_dev->lock. While the lock is definitely needed to protect the setting/unsetting of the KVM pointer, it is not necessarily critical for setting the masks, so this will not be done under protection of the matrix_dev->lock.
Fixes: f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated") Cc: stable@vger.kernel.org Signed-off-by: Tony Krowiak akrowiak@linux.ibm.com
drivers/s390/crypto/vfio_ap_ops.c | 75 ++++++++++++++++++------------- 1 file changed, 45 insertions(+), 30 deletions(-)
static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev) {
- kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
- matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
- vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
- kvm_put_kvm(matrix_mdev->kvm);
- matrix_mdev->kvm = NULL;
- if (matrix_mdev->kvm) {
If you're doing setting/unsetting under matrix_dev->lock, is it possible that matrix_mdev->kvm gets unset between here and the next line, as you don't hold the lock?
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; 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.
kvm_arch_crypto_clear_masks(matrix_mdev->kvm);mutex_lock(&matrix_dev->lock);matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;vfio_ap_mdev_reset_queues(matrix_mdev->mdev);kvm_put_kvm(matrix_mdev->kvm);matrix_mdev->kvm = NULL;mutex_unlock(&matrix_dev->lock);- }
}