virtio-9p: handle handle_9p_output() error

A broken guest may send a request without providing buffers for the reply
or for the request itself, and virtqueue_pop() will return an element with
either in_num == 0 or out_num == 0.

All 9P requests are expected to start with the following 7-byte header:

            uint32_t size_le;
            uint8_t id;
            uint16_t tag_le;

If iov_to_buf() fails to return these 7 bytes, then something is wrong in
the guest.

In both cases, it is wrong to crash QEMU, since the root cause lies in the
guest.

This patch hence does the following:
- keep the check of in_num since pdu_complete() assumes it has enough
  space to store the reply and we will send something broken to the guest
- let iov_to_buf() handle out_num == 0, since it will return 0 just like
  if the guest had provided an zero-sized buffer.
- call virtio_error() to inform the guest that the device is now broken,
  instead of aborting
- detach the request from the virtqueue and free it
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/9pfs/virtio-9p-device.c

@@ -41,6 +41,7 @@ static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
     V9fsState *s = &v->state;
     V9fsPDU *pdu;
     ssize_t len;
+    VirtQueueElement *elem;
 
     while ((pdu = pdu_alloc(s))) {
         struct {
@@ -48,21 +49,28 @@ static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
             uint8_t id;
             uint16_t tag_le;
         } QEMU_PACKED out;
-        VirtQueueElement *elem;
 
         elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
         if (!elem) {
-            pdu_free(pdu);
-            break;
+            goto out_free_pdu;
         }
 
-        BUG_ON(elem->out_num == 0 || elem->in_num == 0);
+        if (elem->in_num == 0) {
+            virtio_error(vdev,
+                         "The guest sent a VirtFS request without space for "
+                         "the reply");
+            goto out_free_req;
+        }
         QEMU_BUILD_BUG_ON(sizeof(out) != 7);
 
         v->elems[pdu->idx] = elem;
         len = iov_to_buf(elem->out_sg, elem->out_num, 0,
                          &out, sizeof(out));
-        BUG_ON(len != sizeof(out));
+        if (len != sizeof(out)) {
+            virtio_error(vdev, "The guest sent a malformed VirtFS request: "
+                         "header size is %zd, should be 7", len);
+            goto out_free_req;
+        }
 
         pdu->size = le32_to_cpu(out.size_le);
 
@@ -72,6 +80,14 @@ static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
         qemu_co_queue_init(&pdu->complete);
         pdu_submit(pdu);
     }
+
+    return;
+
+out_free_req:
+    virtqueue_detach_element(vq, elem, 0);
+    g_free(elem);
+out_free_pdu:
+    pdu_free(pdu);
 }
 
 static uint64_t virtio_9p_get_features(VirtIODevice *vdev, uint64_t features,