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

[Tony Krowiak]

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(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 41fc2e4135fe..f4e19aa2acb9 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -322,6 +322,20 @@ static void vfio_ap_matrix_init(struct ap_config_info *info,
 	matrix->adm_max = info->apxa ? info->Nd : 15;
 }
 
+static bool vfio_ap_mdev_has_crycb(struct ap_matrix_mdev *matrix_mdev)
+{
+	return (matrix_mdev->kvm && matrix_mdev->kvm->arch.crypto.crycbd);
+}
+
+static void vfio_ap_mdev_commit_apcb(struct ap_matrix_mdev *matrix_mdev)
+{
+	if (vfio_ap_mdev_has_crycb(matrix_mdev))
+		kvm_arch_crypto_set_masks(matrix_mdev->kvm,
+					  matrix_mdev->matrix.apm,
+					  matrix_mdev->matrix.aqm,
+					  matrix_mdev->matrix.adm);
+}
+
 static int vfio_ap_mdev_create(struct kobject *kobj, struct mdev_device *mdev)
 {
 	struct ap_matrix_mdev *matrix_mdev;
@@ -1028,7 +1042,9 @@ 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 not be taken prior to
+ * calling this function; doing so may result in a circular lock dependency
+ * when the kvm->lock is taken to set masks in the guest's APCB.
  *
  * Return 0 if no other mediated matrix device has a reference to @kvm;
  * otherwise, returns an -EPERM.
@@ -1038,6 +1054,8 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
 {
 	struct ap_matrix_mdev *m;
 
+	mutex_lock(&matrix_dev->lock);
+
 	list_for_each_entry(m, &matrix_dev->mdev_list, node) {
 		if ((m != matrix_mdev) && (m->kvm == kvm))
 			return -EPERM;
@@ -1046,6 +1064,8 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
 	matrix_mdev->kvm = kvm;
 	kvm_get_kvm(kvm);
 	kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
+	mutex_unlock(&matrix_dev->lock);
+	vfio_ap_mdev_commit_apcb(matrix_mdev);
 
 	return 0;
 }
@@ -1079,13 +1099,27 @@ 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 not be taken prior to calling this
+ * function; doing so may result in a circular lock dependency when the
+ * kvm->lock is taken to clear the masks in the guest's APCB.
+ */
 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) {
+		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);
+	}
 }
 
 static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
@@ -1098,32 +1132,15 @@ static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
 		return NOTIFY_OK;
 
 	matrix_mdev = container_of(nb, struct ap_matrix_mdev, group_notifier);
-	mutex_lock(&matrix_dev->lock);
-
-	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;
 }
 
@@ -1257,10 +1274,8 @@ 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);
-	mutex_unlock(&matrix_dev->lock);
 
 	vfio_unregister_notifier(mdev_dev(mdev), VFIO_IOMMU_NOTIFY,
 				 &matrix_mdev->iommu_notifier);
-- 
2.21.1