[PATCH 3.16 064/192] vfio: Fix group release deadlock

Ben Hutchings
2017-10-09T13:22:37Z

From: Alex Williamson <alex.williamson@redhat.com>

commit 811642d8d8a82c0cce8dc2debfdaf23c5a144839 upstream.

If vfio_iommu_group_notifier() acquires a group reference and that
reference becomes the last reference to the group, then vfio_group_put
introduces a deadlock code path where we're trying to unregister from
the iommu notifier chain from within a callout of that chain. Use a
work_struct to release this reference asynchronously.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/vfio/vfio.c | 37 ++++++++++++++++++++++++++++++++++++-
1 file changed, 36 insertions(+), 1 deletion(-)

--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -278,6 +278,34 @@ static void vfio_group_put(struct vfio_g
kref_put_mutex(&group->kref, vfio_group_release, &vfio.group_lock);
}

+struct vfio_group_put_work {
+ struct work_struct work;
+ struct vfio_group *group;
+};
+
+static void vfio_group_put_bg(struct work_struct *work)
+{
+ struct vfio_group_put_work *do_work;
+
+ do_work = container_of(work, struct vfio_group_put_work, work);
+
+ vfio_group_put(do_work->group);
+ kfree(do_work);
+}
+
+static void vfio_group_schedule_put(struct vfio_group *group)
+{
+ struct vfio_group_put_work *do_work;
+
+ do_work = kmalloc(sizeof(*do_work), GFP_KERNEL);
+ if (WARN_ON(!do_work))
+ return;
+
+ INIT_WORK(&do_work->work, vfio_group_put_bg);
+ do_work->group = group;
+ schedule_work(&do_work->work);
+}
+
/* Assume group_lock or group reference is held */
static void vfio_group_get(struct vfio_group *group)
{
@@ -553,7 +581,14 @@ static int vfio_iommu_group_notifier(str
break;
}

- vfio_group_put(group);
+ /*
+ * If we're the last reference to the group, the group will be
+ * released, which includes unregistering the iommu group notifier.
+ * We hold a read-lock on that notifier list, unregistering needs
+ * a write-lock... deadlock. Release our reference asynchronously
+ * to avoid that situation.
+ */
+ vfio_group_schedule_put(group);
return NOTIFY_OK;
}