[PATCH] vhost: Check for valid vdev in vhost_backend_handle_iotlb_msg

["Eugenio Pérez" <eperezma@redhat.com>]

Not checking this can lead to invalid dev->vdev member access in
vhost_device_iotlb_miss if backend issue an iotlb message in a bad
timing, either maliciously or by a bug.

Reproduced rebooting a guest with testpmd in txonly forward mode.
 #0  0x0000559ffff94394 in vhost_device_iotlb_miss (
     dev=dev@entry=0x55a0012f6680, iova=10245279744, write=1)
     at ../hw/virtio/vhost.c:1013
 #1  0x0000559ffff9ac31 in vhost_backend_handle_iotlb_msg (
     imsg=0x7ffddcfd32c0, dev=0x55a0012f6680)
     at ../hw/virtio/vhost-backend.c:411
 #2  vhost_backend_handle_iotlb_msg (dev=dev@entry=0x55a0012f6680,
     imsg=imsg@entry=0x7ffddcfd32c0)
     at ../hw/virtio/vhost-backend.c:404
 #3  0x0000559fffeded7b in slave_read (opaque=0x55a0012f6680)
     at ../hw/virtio/vhost-user.c:1464
 #4  0x000055a0000c541b in aio_dispatch_handler (
     ctx=ctx@entry=0x55a0010a2120, node=0x55a0012d9e00)
     at ../util/aio-posix.c:329

Fixes: 6dcdd06e3b ("spec/vhost-user spec: Add IOMMU support")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-backend.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
index 222bbcc62d..31b33bde37 100644
--- a/hw/virtio/vhost-backend.c
+++ b/hw/virtio/vhost-backend.c
@@ -406,6 +406,11 @@ int vhost_backend_handle_iotlb_msg(struct vhost_dev *dev,
 {
     int ret = 0;
 
+    if (unlikely(!dev->vdev)) {
+        error_report("Unexpected IOTLB message when virtio device is stopped");
+        return -EINVAL;
+    }
+
     switch (imsg->type) {
     case VHOST_IOTLB_MISS:
         ret = vhost_device_iotlb_miss(dev, imsg->iova,
-- 
2.27.0