[PATCH v2 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 | 119 +++++++++++++++++++++---------
 1 file changed, 84 insertions(+), 35 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 41fc2e4135fe..8574b6ecc9c5 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -1027,8 +1027,21 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
  * @matrix_mdev: a mediated matrix device
  * @kvm: reference to KVM instance
  *
- * Verifies no other mediated matrix device has @kvm and sets a reference to
- * it in @matrix_mdev->kvm.
+ * Sets all data for @matrix_mdev that are needed to manage AP resources
+ * for the guest whose state is represented by @kvm:
+ * 1. Verifies no other mediated device has a reference to @kvm.
+ * 2. Increments the ref count for @kvm so it doesn't disappear until the
+ *    vfio_ap driver is notified the pointer is being nullified.
+ * 3. Sets a reference to the PQAP hook (i.e., handle_pqap() function) into
+ *    @kvm to handle interception of the PQAP(AQIC) instruction.
+ * 4. Sets the masks supplying the AP configuration to the KVM guest.
+ * 5. Sets the KVM pointer into @kvm so the vfio_ap driver can access it.
+ *
+ * Note: The matrix_dev->lock must be taken prior to calling
+ * this function; however, the lock will be temporarily released to avoid a
+ * potential circular lock dependency with other asynchronous processes that
+ * lock the kvm->lock mutex which is also needed to supply the guest's AP
+ * configuration.
  *
  * Return 0 if no other mediated matrix device has a reference to @kvm;
  * otherwise, returns an -EPERM.
@@ -1043,9 +1056,17 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
 			return -EPERM;
 	}
 
-	matrix_mdev->kvm = kvm;
-	kvm_get_kvm(kvm);
-	kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
+	if (kvm->arch.crypto.crycbd) {
+		kvm_get_kvm(kvm);
+		kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
+		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);
+		matrix_mdev->kvm = kvm;
+	}
 
 	return 0;
 }
@@ -1079,51 +1100,80 @@ 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
+ *
+ * Performs clean-up of resources no longer needed by @matrix_mdev.
+ *
+ * Note: The matrix_dev->lock must be taken prior to calling this
+ * function; however,  the lock will be temporarily released to avoid a
+ * potential circular lock dependency with other asynchronous processes that
+ * lock the kvm->lock mutex which is also needed to update the guest's AP
+ * configuration as follows:
+ *	1.  Grab a reference to the KVM pointer stored in @matrix_mdev.
+ *	2.  Set the KVM pointer in @matrix_mdev to NULL so no other asynchronous
+ *	    process uses it (e.g., assign_adapter store function) after
+ *	    unlocking the matrix_dev->lock mutex.
+ *	3.  Set the PQAP hook to NULL so it will not be invoked after unlocking
+ *	    the matrix_dev->lock mutex.
+ *	4.  Unlock the matrix_dev->lock mutex to avoid circular lock
+ *	    dependencies.
+ *	5.  Clear the masks in the guest's APCB to remove guest access to AP
+ *	    resources assigned to @matrix_mdev.
+ *	6.  Lock the matrix_dev->lock mutex to prevent access to resources
+ *	    assigned to @matrix_mdev while the remainder of the cleanup
+ *	    operations take place.
+ *	7.  Decrement the reference counter incremented in #1.
+ *	8.  Set the reference to the KVM pointer grabbed in #1 into @matrix_mdev
+ *	    (set to NULL in #2) because it will be needed when the queues are
+ *	    reset to clean up any IRQ resources being held.
+ *	9.  Decrement the reference count that was incremented when the KVM
+ *	    pointer was originally set by the group notifier.
+ *	10. Set the KVM pointer @matrix_mdev to NULL to prevent its usage from
+ *	    here on out.
+ *
+ */
 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);
+		matrix_mdev->kvm = NULL;
+		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);
+		matrix_mdev->kvm = kvm;
+		vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
+		kvm_put_kvm(matrix_mdev->kvm);
+		matrix_mdev->kvm = NULL;
+	}
 }
 
 static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
 				       unsigned long action, void *data)
 {
-	int ret, notify_rc = NOTIFY_OK;
+	int notify_rc = NOTIFY_OK;
 	struct ap_matrix_mdev *matrix_mdev;
 
 	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 there is no CRYCB pointer, then we can't copy the masks */
-	if (!matrix_mdev->kvm->arch.crypto.crycbd) {
+	if (!data)
+		vfio_ap_mdev_unset_kvm(matrix_mdev);
+	else if (vfio_ap_mdev_set_kvm(matrix_mdev, data))
 		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 +1308,7 @@ 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