bus: Implement GDBusAuthObserver callback

ibus uses a GDBusServer with G_DBUS_SERVER_FLAGS_AUTHENTICATION_ALLOW_ANONYMOUS,
and doesn't set a GDBusAuthObserver, which allows anyone who can connect
to its AF_UNIX socket to authenticate and be authorized to send method calls.
It also seems to use an abstract AF_UNIX socket, which does not have
filesystem permissions, so the practical effect might be that a local
attacker can connect to another user's ibus service and make arbitrary
method calls.

BUGS=rhbz#1717958

bus/server.c

@@ -2,7 +2,8 @@
 /* vim:set et sts=4: */
 /* bus - The Input Bus
  * Copyright (C) 2008-2010 Peng Huang <shawn.p.huang@gmail.com>
- * Copyright (C) 2008-2010 Red Hat, Inc.
+ * Copyright (C) 2011-2019 Takao Fujiwara <takao.fujiwara1@gmail.com>
+ * Copyright (C) 2008-2019 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -69,17 +70,64 @@ _restart_server (void)
     exit (-1);
 }
 
+/**
+ * bus_allow_mechanism_cb:
+ * @observer: A #GDBusAuthObserver.
+ * @mechanism: The name of the mechanism.
+ * @user_data: always %NULL.
+ *
+ * Check if @mechanism can be used to authenticate the other peer.
+ * Returns: %TRUE if the peer's mechanism is allowed.
+ */
+static gboolean
+bus_allow_mechanism_cb (GDBusAuthObserver     *observer,
+                        const gchar           *mechanism,
+                        G_GNUC_UNUSED gpointer user_data)
+{
+    if (g_strcmp0 (mechanism, "EXTERNAL") == 0)
+        return TRUE;
+    return FALSE;
+}
+
+/**
+ * bus_authorize_authenticated_peer_cb:
+ * @observer: A #GDBusAuthObserver.
+ * @stream: A #GIOStream.
+ * @credentials: A #GCredentials.
+ * @user_data: always %NULL.
+ *
+ * Check if a peer who has already authenticated should be authorized.
+ * Returns: %TRUE if the peer's credential is authorized.
+ */
+static gboolean
+bus_authorize_authenticated_peer_cb (GDBusAuthObserver     *observer,
+                                     GIOStream             *stream,
+                                     GCredentials          *credentials,
+                                     G_GNUC_UNUSED gpointer user_data)
+{
+    gboolean authorized = FALSE;
+    if (credentials) {
+        GCredentials *own_credentials = g_credentials_new ();
+        if (g_credentials_is_same_user (credentials, own_credentials, NULL))
+            authorized = TRUE;
+        g_object_unref (own_credentials);
+    }
+    return authorized;
+}
+
 /**
  * bus_new_connection_cb:
- * @user_data: always NULL.
- * @returns: TRUE when the function can handle the connection.
+ * @observer: A #GDBusAuthObserver.
+ * @dbus_connection: A #GDBusconnection.
+ * @user_data: always %NULL.
  *
  * Handle incoming connections.
+ * Returns: %TRUE when the function can handle the connection.
  */
 static gboolean
-bus_new_connection_cb (GDBusServer     *server,
-                       GDBusConnection *dbus_connection,
-                       gpointer         user_data)
+bus_new_connection_cb (GDBusServer           *server,
+                       GDBusConnection       *dbus_connection,
+                       G_GNUC_UNUSED gpointer user_data)
 {
     BusConnection *connection = bus_connection_new (dbus_connection);
     bus_dbus_impl_new_connection (dbus, connection);
@@ -94,9 +142,9 @@ bus_new_connection_cb (GDBusServer     *server,
 }
 
 static void
-_server_connect_start_portal_cb (GObject      *source_object,
-                                 GAsyncResult *res,
-                                 gpointer      user_data)
+_server_connect_start_portal_cb (GObject               *source_object,
+                                 GAsyncResult          *res,
+                                 G_GNUC_UNUSED gpointer user_data)
 {
     GVariant *result;
     GError *error = NULL;
@@ -113,9 +161,9 @@ _server_connect_start_portal_cb (GObject      *source_object,
 }
 
 static void
-bus_acquired_handler (GDBusConnection *connection,
-                      const gchar     *name,
-                      gpointer         user_data)
+bus_acquired_handler (GDBusConnection       *connection,
+                      const gchar           *name,
+                      G_GNUC_UNUSED gpointer user_data)
 {
     g_dbus_connection_call (connection,
                             IBUS_SERVICE_PORTAL,
@@ -136,14 +184,17 @@ void
 bus_server_init (void)
 {
     GError *error = NULL;
+    GDBusServerFlags flags = G_DBUS_SERVER_FLAGS_NONE;
+    gchar *guid;
+    GDBusAuthObserver *observer;
 
     dbus = bus_dbus_impl_get_default ();
     ibus = bus_ibus_impl_get_default ();
     bus_dbus_impl_register_object (dbus, (IBusService *)ibus);
 
     /* init server */
-    GDBusServerFlags flags = G_DBUS_SERVER_FLAGS_AUTHENTICATION_ALLOW_ANONYMOUS;
-    gchar *guid = g_dbus_generate_guid ();
+    guid = g_dbus_generate_guid ();
+    observer = g_dbus_auth_observer_new ();
     if (!g_str_has_prefix (g_address, "unix:tmpdir=") &&
         !g_str_has_prefix (g_address, "unix:path=")) {
         g_error ("Your socket address does not have the format unix:tmpdir=$DIR "
@@ -152,7 +203,7 @@ bus_server_init (void)
     server =  g_dbus_server_new_sync (
                     g_address, /* the place where the socket file lives, e.g. /tmp, abstract namespace, etc. */
                     flags, guid,
-                    NULL /* observer */,
+                    observer,
                     NULL /* cancellable */,
                     &error);
     if (server == NULL) {
@@ -162,7 +213,13 @@ bus_server_init (void)
     }
     g_free (guid);
 
-    g_signal_connect (server, "new-connection", G_CALLBACK (bus_new_connection_cb), NULL);
+    g_signal_connect (observer, "allow-mechanism",
+                      G_CALLBACK (bus_allow_mechanism_cb), NULL);
+    g_signal_connect (observer, "authorize-authenticated-peer",
+                      G_CALLBACK (bus_authorize_authenticated_peer_cb), NULL);
+    g_object_unref (observer);
+    g_signal_connect (server, "new-connection",
+                      G_CALLBACK (bus_new_connection_cb), NULL);
 
     g_dbus_server_start (server);