Since disconnect can happen at any time during initialization not all
vring buffers (for instance used vring) can be intialized successfully.
If the buffer was not initialized then vhost_memory_unmap call will lead
to SIGSEGV. Add checks for the vring address value before calling unmap.
Also add assert() in the vhost_memory_unmap() routine.

Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
---
 hw/virtio/vhost.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index ddbdc53..3ee50c4 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -314,6 +314,8 @@ static void vhost_memory_unmap(struct vhost_dev *dev, void *buffer,
                                hwaddr len, int is_write,
                                hwaddr access_len)
 {
+    assert(buffer);
+
     if (!vhost_dev_has_iommu(dev)) {
         cpu_physical_memory_unmap(buffer, len, is_write, access_len);
     }
@@ -1132,12 +1134,25 @@ static void vhost_virtqueue_stop(struct vhost_dev *dev,
                                                 vhost_vq_index);
     }
 
-    vhost_memory_unmap(dev, vq->used, virtio_queue_get_used_size(vdev, idx),
-                       1, virtio_queue_get_used_size(vdev, idx));
-    vhost_memory_unmap(dev, vq->avail, virtio_queue_get_avail_size(vdev, idx),
-                       0, virtio_queue_get_avail_size(vdev, idx));
-    vhost_memory_unmap(dev, vq->desc, virtio_queue_get_desc_size(vdev, idx),
-                       0, virtio_queue_get_desc_size(vdev, idx));
+    /*
+     * Since the vhost-user disconnect can happen during initialization
+     * check if vring was initialized, before making unmap.
+     */
+    if (vq->used) {
+        vhost_memory_unmap(dev, vq->used,
+                           virtio_queue_get_used_size(vdev, idx),
+                           1, virtio_queue_get_used_size(vdev, idx));
+    }
+    if (vq->avail) {
+        vhost_memory_unmap(dev, vq->avail,
+                           virtio_queue_get_avail_size(vdev, idx),
+                           0, virtio_queue_get_avail_size(vdev, idx));
+    }
+    if (vq->desc) {
+        vhost_memory_unmap(dev, vq->desc,
+                           virtio_queue_get_desc_size(vdev, idx),
+                           0, virtio_queue_get_desc_size(vdev, idx));
+    }
 }
 
 static void vhost_eventfd_add(MemoryListener *listener,
-- 
2.7.4