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

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On 2/10/21 10:24 AM, Halil Pasic wrote:
On Wed, 10 Feb 2021 11:53:34 +0100
Cornelia Huck <cohuck@xxxxxxxxxx> wrote:

On Tue,  9 Feb 2021 14:48:30 -0500
Tony Krowiak <akrowiak@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
---
  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.

How ironic, that is exactly what I did after agreeing with Connie.


+		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);
+	}
  }




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