webrtc: Fix possible use-after-free of GstWebRTCICETransport

Because of the asynchronous resolving of mDNS ICE candidates it is
possible that GstWebRTCICE outlives webrtcbin. This in turn prolongs
the lifetime of the GstWebRTCNiceStream objects via refs in
nice_stream_map. Thus the GstWebRTCICETransport objects held in
GstWebRTCNiceStream may be invalid at the time they are accessed by
the _on_candidate_gathering_done() callback since GstWebRTCNiceStream
doesn't take a reference to them. Doing so would create a circular
reference, so instead this commit introduces weak references to the
transport objects and then we can check if the objects are valid before
accessing them.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3502>

subprojects/gst-plugins-bad/gst-libs/gst/webrtc/nice/nicestream.c

@@ -80,6 +80,21 @@ gst_webrtc_nice_stream_get_property (GObject * object, guint prop_id,
   }
 }
 
+static GWeakRef *
+weak_new (gpointer object)
+{
+  GWeakRef *weak = g_new0 (GWeakRef, 1);
+  g_weak_ref_init (weak, object);
+  return weak;
+}
+
+static void
+weak_free (GWeakRef * weak)
+{
+  g_weak_ref_clear (weak);
+  g_free (weak);
+}
+
 static void
 gst_webrtc_nice_stream_finalize (GObject * object)
 {
@@ -99,6 +114,7 @@ gst_webrtc_nice_stream_finalize (GObject * object)
     gst_object_unref (ice);
   }
 
+  g_list_foreach (stream->priv->transports, (GFunc) weak_free, NULL);
   g_list_free (stream->priv->transports);
   stream->priv->transports = NULL;
 
@@ -107,6 +123,15 @@ gst_webrtc_nice_stream_finalize (GObject * object)
   G_OBJECT_CLASS (parent_class)->finalize (object);
 }
 
+static GList *
+_delete_transport (GList ** transports, GList * link)
+{
+  GList *next = link->next;
+  weak_free (link->data);
+  *transports = g_list_delete_link (*transports, link);
+  return next;
+}
+
 static void
 _on_candidate_gathering_done (NiceAgent * agent, guint stream_id,
     GWeakRef * ice_weak)
@@ -125,10 +150,15 @@ _on_candidate_gathering_done (NiceAgent * agent, guint stream_id,
   ice->priv->gathered = TRUE;
 
   for (l = ice->priv->transports; l; l = l->next) {
-    GstWebRTCICETransport *ice = l->data;
+    GstWebRTCICETransport *trans = g_weak_ref_get (l->data);
 
-    gst_webrtc_ice_transport_gathering_state_change (ice,
+    if (trans) {
+      gst_webrtc_ice_transport_gathering_state_change (trans,
           GST_WEBRTC_ICE_GATHERING_STATE_COMPLETE);
+      g_object_unref (trans);
+    } else {
+      l = _delete_transport (&ice->priv->transports, l);
+    }
   }
 
 cleanup:
@@ -145,37 +175,28 @@ gst_webrtc_nice_stream_find_transport (GstWebRTCICEStream * stream,
   GstWebRTCNiceStream *nice_stream = GST_WEBRTC_NICE_STREAM (stream);
 
   for (l = nice_stream->priv->transports; l; l = l->next) {
-    GstWebRTCICETransport *trans = l->data;
+    GstWebRTCICETransport *trans = g_weak_ref_get (l->data);
+    if (trans) {
       g_object_get (trans, "component", &trans_comp, NULL);
 
       if (component == trans_comp)
-      return gst_object_ref (trans);
+        return trans;
+      else
+        gst_object_unref (trans);
+    } else {
+      l = _delete_transport (&nice_stream->priv->transports, l);
+    }
   }
 
   ret =
       GST_WEBRTC_ICE_TRANSPORT (gst_webrtc_nice_transport_new (nice_stream,
           component));
   nice_stream->priv->transports =
-      g_list_prepend (nice_stream->priv->transports, ret);
+      g_list_prepend (nice_stream->priv->transports, weak_new (ret));
 
   return ret;
 }
 
-static GWeakRef *
-weak_new (GstWebRTCNiceStream * stream)
-{
-  GWeakRef *weak = g_new0 (GWeakRef, 1);
-  g_weak_ref_init (weak, stream);
-  return weak;
-}
-
-static void
-weak_free (GWeakRef * weak)
-{
-  g_weak_ref_clear (weak);
-  g_free (weak);
-}
-
 static void
 gst_webrtc_nice_stream_constructed (GObject * object)
 {
@@ -214,10 +235,15 @@ gst_webrtc_nice_stream_gather_candidates (GstWebRTCICEStream * stream)
     return TRUE;
 
   for (l = nice_stream->priv->transports; l; l = l->next) {
-    GstWebRTCICETransport *trans = l->data;
+    GstWebRTCICETransport *trans = g_weak_ref_get (l->data);
 
+    if (trans) {
       gst_webrtc_ice_transport_gathering_state_change (trans,
           GST_WEBRTC_ICE_GATHERING_STATE_GATHERING);
+      g_object_unref (trans);
+    } else {
+      l = _delete_transport (&nice_stream->priv->transports, l);
+    }
   }
 
   ice = GST_WEBRTC_ICE (g_weak_ref_get (&nice_stream->priv->ice_weak));
@@ -248,9 +274,14 @@ gst_webrtc_nice_stream_gather_candidates (GstWebRTCICEStream * stream)
   }
 
   for (l = nice_stream->priv->transports; l; l = l->next) {
-    GstWebRTCNiceTransport *trans = l->data;
+    GstWebRTCNiceTransport *trans = g_weak_ref_get (l->data);
 
+    if (trans) {
       gst_webrtc_nice_transport_update_buffer_size (trans);
+      g_object_unref (trans);
+    } else {
+      l = _delete_transport (&nice_stream->priv->transports, l);
+    }
   }
 
 cleanup: