[PATCH v20 09/20] s390/vfio-ap: use proper locking order when setting/clearing KVM pointer

Tony Krowiak
2022-06-21T15:52:23Z

The group notifier that handles the VFIO_GROUP_NOTIFY_SET_KVM event must
use the required locks in proper locking order to dynamically update the
guest's APCB. The proper locking order is:

1. matrix_dev->guests_lock: required to use the KVM pointer to
update a KVM guest's APCB.

2. matrix_mdev->kvm->lock: required to update a KVM guest's APCB.

3. matrix_dev->mdevs_lock: required to store or access the data
stored in a struct ap_matrix_mdev instance.

Two macros are introduced to acquire and release the locks in the proper
order. These macros are now used by the group notifier functions.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
---
drivers/s390/crypto/vfio_ap_ops.c | 56 +++++++++++++++++++++++++------
1 file changed, 46 insertions(+), 10 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index f81bf8686464..f31db1248740 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -34,6 +34,47 @@ static int vfio_ap_mdev_reset_queues(struct ap_matrix_mdev *matrix_mdev);
static struct vfio_ap_queue *vfio_ap_find_queue(int apqn);
static const struct vfio_device_ops vfio_ap_matrix_dev_ops;

+/**
+ * get_update_locks_for_kvm: Acquire the locks required to dynamically update a
+ * KVM guest's APCB in the proper order.
+ *
+ * @kvm: a pointer to a struct kvm object containing the KVM guest's APCB.
+ *
+ * The proper locking order is:
+ * 1. matrix_dev->guests_lock: required to use the KVM pointer to update a KVM
+ * guest's APCB.
+ * 2. kvm->lock: required to update a guest's APCB
+ * 3. matrix_dev->mdevs_lock: required to access data stored in a matrix_mdev
+ *
+ * Note: If @kvm is NULL, the KVM lock will not be taken.
+ */
+#define get_update_locks_for_kvm(kvm) ({ \
+ mutex_lock(&matrix_dev->guests_lock); \
+ if (kvm) \
+ mutex_lock(&kvm->lock); \
+ mutex_lock(&matrix_dev->mdevs_lock); \
+})
+
+/**
+ * release_update_locks_for_kvm: Release the locks used to dynamically update a
+ * KVM guest's APCB in the proper order.
+ *
+ * @kvm: a pointer to a struct kvm object containing the KVM guest's APCB.
+ *
+ * The proper unlocking order is:
+ * 1. matrix_dev->mdevs_lock
+ * 2. kvm->lock
+ * 3. matrix_dev->guests_lock
+ *
+ * Note: If @kvm is NULL, the KVM lock will not be released.
+ */
+#define release_update_locks_for_kvm(kvm) ({ \
+ mutex_unlock(&matrix_dev->mdevs_lock); \
+ if (kvm) \
+ mutex_unlock(&kvm->lock); \
+ mutex_unlock(&matrix_dev->guests_lock); \
+})
+
/**
* vfio_ap_mdev_get_queue - retrieve a queue with a specific APQN from a
* hash table of queues assigned to a matrix mdev
@@ -1266,13 +1307,11 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
up_write(&kvm->arch.crypto.pqap_hook_rwsem);

- mutex_lock(&kvm->lock);
- mutex_lock(&matrix_dev->mdevs_lock);
+ get_update_locks_for_kvm(kvm);

list_for_each_entry(m, &matrix_dev->mdev_list, node) {
if (m != matrix_mdev && m->kvm == kvm) {
- mutex_unlock(&kvm->lock);
- mutex_unlock(&matrix_dev->mdevs_lock);
+ release_update_locks_for_kvm(kvm);
return -EPERM;
}
}
@@ -1283,8 +1322,7 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
matrix_mdev->shadow_apcb.aqm,
matrix_mdev->shadow_apcb.adm);

- mutex_unlock(&kvm->lock);
- mutex_unlock(&matrix_dev->mdevs_lock);
+ release_update_locks_for_kvm(kvm);
}

return 0;
@@ -1335,16 +1373,14 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
kvm->arch.crypto.pqap_hook = NULL;
up_write(&kvm->arch.crypto.pqap_hook_rwsem);

- mutex_lock(&kvm->lock);
- mutex_lock(&matrix_dev->mdevs_lock);
+ get_update_locks_for_kvm(kvm);

kvm_arch_crypto_clear_masks(kvm);
vfio_ap_mdev_reset_queues(matrix_mdev);
kvm_put_kvm(kvm);
matrix_mdev->kvm = NULL;

- mutex_unlock(&kvm->lock);
- mutex_unlock(&matrix_dev->mdevs_lock);
+ release_update_locks_for_kvm(kvm);
}
}

--
2.35.3