vdpa: Fix possible use-after-free for VirtQueueElement

QEMU uses vhost_handle_guest_kick() to forward guest's available
buffers to the vdpa device in SVQ avail ring.

In vhost_handle_guest_kick(), a `g_autofree` `elem` is used to
iterate through the available VirtQueueElements. This `elem` is
then passed to `svq->ops->avail_handler`, specifically to the
vhost_vdpa_net_handle_ctrl_avail(). If this handler fails to
process the CVQ command, vhost_handle_guest_kick() regains
ownership of the `elem`, and either frees it or requeues it.

Yet the problem is that, vhost_vdpa_net_handle_ctrl_avail()
mistakenly frees the `elem`, even if it fails to forward the
CVQ command to vdpa device. This can result in a use-after-free
for the `elem` in vhost_handle_guest_kick().

This patch solves this problem by refactoring
vhost_vdpa_net_handle_ctrl_avail() to only freeing the `elem` if
it owns it.

Fixes: bd907ae ("vdpa: manual forward CVQ buffers")
Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Message-Id: <e3f2d7db477734afe5c6a5ab3fa8b8317514ea34.1688746840.git.yin31149@gmail.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

net/vhost-vdpa.c

@@ -831,7 +831,16 @@ static int vhost_vdpa_net_handle_ctrl_avail(VhostShadowVirtqueue *svq,
         error_report("Bad device CVQ written length");
     }
     vhost_svq_push_elem(svq, elem, MIN(in_len, sizeof(status)));
-    g_free(elem);
+    /*
+     * `elem` belongs to vhost_vdpa_net_handle_ctrl_avail() only when
+     * the function successfully forwards the CVQ command, indicated
+     * by a non-negative value of `dev_written`. Otherwise, it still
+     * belongs to SVQ.
+     * This function should only free the `elem` when it owns.
+     */
+    if (dev_written >= 0) {
+        g_free(elem);
+    }
     return dev_written < 0 ? dev_written : 0;
 }