[PATCH v2 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks

[Tony Krowiak <akrowiak@xxxxxxxxxxxxx>]

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 | 78 +++++++++++++++++++------------
 1 file changed, 48 insertions(+), 30 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 41fc2e4135fe..bba0f64aa1f7 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -1028,7 +1028,10 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
  * @kvm: reference to KVM instance
  *
  * Verifies no other mediated matrix device has @kvm and sets a reference to
- * it in @matrix_mdev->kvm.
+ * it in @matrix_mdev->kvm. The matrix_dev->lock must be taken prior to calling
+ * this function; however, the lock will be temporarily released while updating
+ * the guest's APCB to avoid a potential circular lock dependency with other
+ * asynchronous processes.
  *
  * Return 0 if no other mediated matrix device has a reference to @kvm;
  * otherwise, returns an -EPERM.
@@ -1043,10 +1046,19 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
 			return -EPERM;
 	}
 
-	matrix_mdev->kvm = kvm;
 	kvm_get_kvm(kvm);
+	matrix_mdev->kvm = kvm;
 	kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
 
+	if (matrix_mdev->kvm && matrix_mdev->kvm->arch.crypto.crycbd) {
+		mutex_unlock(&matrix_dev->lock);
+		kvm_arch_crypto_set_masks(kvm,
+					  matrix_mdev->matrix.apm,
+					  matrix_mdev->matrix.aqm,
+					  matrix_mdev->matrix.adm);
+		mutex_lock(&matrix_dev->lock);
+	}
+
 	return 0;
 }
 
@@ -1079,13 +1091,34 @@ static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
 	return NOTIFY_DONE;
 }
 
+/**
+ * vfio_ap_mdev_unset_kvm
+ *
+ * @matrix_mdev: a matrix mediated device
+ *
+ * Clears the masks in the guest's APCB as well as the reference to KVM from
+ * @matrix_mdev. The matrix_dev->lock must be taken prior to calling this
+ * function; however, the lock will be temporarily released while updating
+ * the guest's APCB to avoid a potential circular lock dependency with other
+ * asynchronous processes.
+ */
 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;
+	struct kvm *kvm;
+
+	if (matrix_mdev->kvm) {
+		kvm = matrix_mdev->kvm;
+		kvm_get_kvm(kvm);
+		kvm->arch.crypto.pqap_hook = NULL;
+		mutex_unlock(&matrix_dev->lock);
+		kvm_arch_crypto_clear_masks(kvm);
+		mutex_lock(&matrix_dev->lock);
+		kvm_put_kvm(kvm);
+		vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
+		if (matrix_mdev->kvm)
+			kvm_put_kvm(matrix_mdev->kvm);
+		matrix_mdev->kvm = NULL;
+	}
 }
 
 static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
@@ -1097,33 +1130,19 @@ static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
 	if (action != VFIO_GROUP_NOTIFY_SET_KVM)
 		return NOTIFY_OK;
 
-	matrix_mdev = container_of(nb, struct ap_matrix_mdev, group_notifier);
 	mutex_lock(&matrix_dev->lock);
+	matrix_mdev = container_of(nb, struct ap_matrix_mdev, group_notifier);
 
-	if (!data) {
-		if (matrix_mdev->kvm)
-			vfio_ap_mdev_unset_kvm(matrix_mdev);
-		goto notify_done;
-	}
-
-	ret = vfio_ap_mdev_set_kvm(matrix_mdev, data);
-	if (ret) {
-		notify_rc = NOTIFY_DONE;
-		goto notify_done;
-	}
+	if (!data)
+		vfio_ap_mdev_unset_kvm(matrix_mdev);
+	else
+		ret = vfio_ap_mdev_set_kvm(matrix_mdev, data);
 
-	/* If there is no CRYCB pointer, then we can't copy the masks */
-	if (!matrix_mdev->kvm->arch.crypto.crycbd) {
+	if (ret)
 		notify_rc = NOTIFY_DONE;
-		goto notify_done;
-	}
-
-	kvm_arch_crypto_set_masks(matrix_mdev->kvm, matrix_mdev->matrix.apm,
-				  matrix_mdev->matrix.aqm,
-				  matrix_mdev->matrix.adm);
 
-notify_done:
 	mutex_unlock(&matrix_dev->lock);
+
 	return notify_rc;
 }
 
@@ -1258,8 +1277,7 @@ static void vfio_ap_mdev_release(struct mdev_device *mdev)
 	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
 
 	mutex_lock(&matrix_dev->lock);
-	if (matrix_mdev->kvm)
-		vfio_ap_mdev_unset_kvm(matrix_mdev);
+	vfio_ap_mdev_unset_kvm(matrix_mdev);
 	mutex_unlock(&matrix_dev->lock);
 
 	vfio_unregister_notifier(mdev_dev(mdev), VFIO_IOMMU_NOTIFY,
-- 
2.21.1