[v3] virtio: guard vring access when setting notification

Commit Message

Cornelia Huck March 1, 2017, 5:58 p.m. UTC

Switching to vring caches exposed an existing bug in
virtio_queue_set_notification(): We can't access vring structures
if they have not been set up yet. This may happen, for example,
for virtio-blk devices with multiple queues: The code will try to
switch notifiers for every queue, but the guest may have only set up
a subset of them.

Fix this by guarding access to the vring memory by checking for
vring.desc. The first aio poll will iron out any remaining
inconsistencies for later-configured queues (buggy legacy drivers).

Signed-off-by: Cornelia Huck <[e-mail]>
---
v2->v3:
- Switch to a very simple check for !desc, which should be fine for
  the existing users.

We might want to assert that callers don't start aio before DRIVER_OK
resp. first kick, but that can wait until after we've fixed this bug.
---
 hw/virtio/virtio.c | 4 ++++
 1 file changed, 4 insertions(+)

Patch

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index e487e36..bf8a644 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -288,6 +288,10 @@  void virtio_queue_set_notification(VirtQueue *vq, int enable)
 {
     vq->notification = enable;
 
+    if (!vq->vring.desc) {
+        return;
+    }
+
     rcu_read_lock();
     if (virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
         vring_set_avail_event(vq, vring_avail_idx(vq));