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

Tony Krowiak
2021-02-09T20:05:46Z

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(-)

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