wayland: Add API to launch trusted clients

Allowing code from inside mutter to create a child process and
delegate on it some of its tasks is something very useful. This can
be done easily with the g_subprocess and g_subprocess_launcher classes
already available in GLib and GObject.

Unfortunately, although the child process can be a graphical program,
currently it is not possible for the inner code to identify the
windows created by the child in a secure manner (this is: being able
to ensure that a malicious program won't be able to trick the inner
code into thinking it is a child process launched by it).

Under X11 this is not a problem because any program has full control
over their windows, but under Wayland it is a different story: a
program can't neither force their window to be kept at the top (like a
docker program does) or at the bottom (like a program for desktop icons
does), nor hide it from the list of windows. This means that it is not
possible for a "classic", non-priviledged program, to fulfill these
tasks, and it can be done only from code inside mutter (like a
gnome-shell extension).

This is a non desirable situation, because an extension runs in the
same main loop than the whole desktop itself, which means that a
complex extension can need to do too much work inside the main loop,
and freeze the whole desktop for too much time. Also, it is important
to note that javascript doesn't have access to fork(), or threads,
which means that, at most, all the parallel computing that can do is
those available in the _async calls in GLib/GObject.

Also, having to create an extension for any priviledged graphical
element is an stopper for a lot of programmers who already know
GTK+ but doesn't know Clutter.

This patch wants to offer a solution to this problem, by offering a
new class that allows to launch a trusted child process from inside
mutter, and make it to use an specific UNIX socket to communicate
with the compositor. It also allows to check whether an specific
MetaWindow was created by one of this trusted child processes or not.

This allows to create extensions that launch a child process, and
when that process creates a window, the extension can confirm in a
secure way that the window really belongs to that process
launched by it, so it can give to that window "superpowers" like
being kept at the bottom of the desktop, not being listed in the
list of windows or shown in the Activities panel... Also, in future
versions, it could easily implement protocol extensions that only
could be used by these trusted child processes.

Several examples of the usefulness of this are that, with it, it
is possible to write programs that implements:

- desktop icons
- a dock
- a top or bottom bar
...

all in a secure manner, avoiding insecure programs to do the same.
In fact, even if the same code is launched manually, it won't have
those privileges, only the specific process launched from inside
mutter.

Since this is only needed under Wayland, it won't work under X11.

Fixes https://gitlab.gnome.org/GNOME/mutter/issues/741

src/core/window-private.h

@@ -362,6 +362,7 @@ struct _MetaWindow
   /* Computed whether to skip taskbar or not */
   guint skip_taskbar : 1;
   guint skip_pager : 1;
+  guint skip_from_window_list : 1;
 
   /* TRUE if client set these */
   guint wm_state_above : 1;

src/core/window.c

@@ -730,7 +730,8 @@ is_desktop_or_dock_foreach (MetaWindow *window,
 
   *result =
     window->type == META_WINDOW_DESKTOP ||
-    window->type == META_WINDOW_DOCK;
+    window->type == META_WINDOW_DOCK ||
+    window->skip_from_window_list;
   if (*result)
     return FALSE; /* stop as soon as we find one */
   else
@@ -1137,6 +1138,7 @@ _meta_window_shared_new (MetaDisplay         *display,
 
   window->skip_taskbar = FALSE;
   window->skip_pager = FALSE;
+  window->skip_from_window_list = FALSE;
   window->wm_state_above = FALSE;
   window->wm_state_below = FALSE;
   window->wm_state_demands_attention = FALSE;
@@ -5609,15 +5611,15 @@ meta_window_recalc_skip_features (MetaWindow *window)
       if (window->transient_for != NULL)
         window->skip_taskbar = TRUE;
       else
-        window->skip_taskbar = FALSE;
+        window->skip_taskbar = window->skip_from_window_list;
       break;
 
     case META_WINDOW_NORMAL:
       {
         gboolean skip_taskbar_hint, skip_pager_hint;
         meta_window_get_default_skip_hints (window, &skip_taskbar_hint, &skip_pager_hint);
-        window->skip_taskbar = skip_taskbar_hint;
+        window->skip_taskbar = skip_taskbar_hint | window->skip_from_window_list;
-        window->skip_pager = skip_pager_hint;
+        window->skip_pager = skip_pager_hint | window->skip_from_window_list;
       }
       break;
     }

src/meson.build

@@ -504,6 +504,7 @@ if have_wayland
     'wayland/meta-wayland-buffer.c',
     'wayland/meta-wayland-buffer.h',
     'wayland/meta-wayland.c',
+    'wayland/meta-wayland-client.c',
     'wayland/meta-wayland-cursor-surface.c',
     'wayland/meta-wayland-cursor-surface.h',
     'wayland/meta-wayland-data-device.c',

src/meta/meson.build

@@ -45,6 +45,12 @@ mutter_public_headers = [
   'workspace.h',
 ]
 
+if have_wayland
+  mutter_public_headers += [
+    'meta-wayland-client.h',
+  ]
+endif
+
 if have_x11
   mutter_public_headers += [
     'meta-x11-display.h',

src/meta/meta-wayland-client.h

@@ -0,0 +1,67 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+
+/*
+ * Copyright 2019 Sergio Costas (rastersoft@gmail.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+#ifndef META_WAYLAND_CLIENT_H
+#define META_WAYLAND_CLIENT_H
+
+#include <glib-object.h>
+#include <gio/gio.h>
+
+#include "meta/display.h"
+
+G_BEGIN_DECLS
+
+#define META_TYPE_WAYLAND_CLIENT (meta_wayland_client_get_type ())
+META_EXPORT
+G_DECLARE_FINAL_TYPE (MetaWaylandClient, meta_wayland_client, META, WAYLAND_CLIENT, GObject)
+
+META_EXPORT
+MetaWaylandClient *meta_wayland_client_new (GSubprocessLauncher  *launcher,
+                                            GError              **error);
+
+META_EXPORT
+GSubprocess *meta_wayland_client_spawn (MetaWaylandClient  *client,
+                                        MetaDisplay        *display,
+                                        GError            **error,
+                                        const char         *argv0,
+                                        ...) G_GNUC_NULL_TERMINATED;
+
+META_EXPORT
+GSubprocess *meta_wayland_client_spawnv (MetaWaylandClient   *client,
+                                         MetaDisplay         *display,
+                                         const char * const  *argv,
+                                         GError             **error);
+
+META_EXPORT
+gboolean meta_wayland_client_owns_window (MetaWaylandClient *client,
+                                          MetaWindow        *window);
+
+META_EXPORT
+void meta_wayland_client_hide_from_window_list (MetaWaylandClient *client,
+                                                MetaWindow        *window);
+
+META_EXPORT
+void meta_wayland_client_show_in_window_list (MetaWaylandClient *client,
+                                              MetaWindow        *window);
+
+G_END_DECLS
+
+#endif /* META_WAYLAND_CLIENT_H */

src/wayland/meta-wayland-client.c

@@ -0,0 +1,332 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+
+/*
+ * Copyright 2019 Sergio Costas (rastersoft@gmail.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+/**
+ * SECTION: meta-wayland-client
+ * @title MetaWaylandClient
+ * @include: gio/gsubprocess.h
+ * A class that allows to launch a trusted client and detect if an specific
+ * Wayland window belongs to it.
+ */
+
+#include "config.h"
+
+#include "meta/meta-wayland-client.h"
+
+#include <gio/gio.h>
+#include <glib-object.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <wayland-server.h>
+
+#include "core/window-private.h"
+#include "meta/util.h"
+#include "wayland/meta-wayland-private.h"
+#include "wayland/meta-wayland-types.h"
+
+struct _MetaWaylandClient
+{
+  GObject parent_instance;
+
+  GSubprocessLauncher *launcher;
+  GSubprocess *subprocess;
+  GCancellable *died_cancellable;
+  gboolean process_running;
+  struct wl_client *wayland_client;
+};
+
+G_DEFINE_TYPE (MetaWaylandClient, meta_wayland_client, G_TYPE_OBJECT)
+
+static void
+meta_wayland_client_dispose (GObject *object)
+{
+  MetaWaylandClient *client = META_WAYLAND_CLIENT (object);
+
+  g_cancellable_cancel (client->died_cancellable);
+  g_clear_object (&client->died_cancellable);
+  g_clear_object (&client->launcher);
+  g_clear_object (&client->subprocess);
+
+  G_OBJECT_CLASS (meta_wayland_client_parent_class)->dispose (object);
+}
+
+static void
+meta_wayland_client_class_init (MetaWaylandClientClass *klass)
+{
+  GObjectClass *object_class = G_OBJECT_CLASS (klass);
+
+  object_class->dispose = meta_wayland_client_dispose;
+}
+
+static void
+meta_wayland_client_init (MetaWaylandClient *client)
+{
+}
+
+static void
+process_died (GObject      *source,
+              GAsyncResult *result,
+              gpointer      user_data)
+{
+  MetaWaylandClient *client = META_WAYLAND_CLIENT (user_data);
+
+  client->process_running = FALSE;
+}
+
+/**
+ * meta_wayland_client_new:
+ * @launcher: (not nullable): a GSubprocessLauncher to use to launch the subprocess
+ * @error: (nullable): Error
+ *
+ * Creates a new #MetaWaylandClient. The GSubprocesslauncher passed is
+ * stored internally and will be used to launch the subprocess.
+ *
+ * Returns: A #MetaWaylandClient or %NULL if %error is set. Free with
+ * g_object_unref().
+ */
+MetaWaylandClient *
+meta_wayland_client_new (GSubprocessLauncher  *launcher,
+                         GError              **error)
+{
+  MetaWaylandClient *client;
+
+  g_return_val_if_fail (error == NULL || *error == NULL, NULL);
+
+  if (!meta_is_wayland_compositor ())
+    {
+      g_set_error (error,
+                   G_IO_ERROR,
+                   G_IO_ERROR_NOT_SUPPORTED,
+                   "MetaWaylandClient can be used only with Wayland.");
+      return NULL;
+    }
+
+  if (launcher == NULL)
+    {
+      g_set_error (error,
+                   G_IO_ERROR,
+                   G_IO_ERROR_INVALID_ARGUMENT,
+                   "Invalid launcher.");
+      return NULL;
+    }
+
+  client = g_object_new (META_TYPE_WAYLAND_CLIENT, NULL);
+  client->launcher = launcher;
+  return client;
+}
+
+/**
+ * meta_wayland_client_spawnv:
+ * @client: a #MetaWaylandClient
+ * @display: (not nullable): the current MetaDisplay
+ * @argv: (array zero-terminated=1) (element-type filename): Command line arguments
+ * @error: (nullable): Error
+ *
+ * Creates a #GSubprocess given a provided array of arguments, launching a new
+ * process with the binary specified in the first element of argv, and with the
+ * rest of elements as parameters. It also sets up a new Wayland socket and sets
+ * the environment variable WAYLAND_SOCKET to make the new process to use it.
+ *
+ * Returns: (transfer full): A new #GSubprocess, or %NULL on error (and @error
+ * will be set)
+ **/
+GSubprocess *
+meta_wayland_client_spawnv (MetaWaylandClient   *client,
+                            MetaDisplay         *display,
+                            const char * const  *argv,
+                            GError             **error)
+{
+  int client_fd[2];
+  GSubprocess *subprocess;
+  struct wl_client *wayland_client;
+  MetaWaylandCompositor *compositor;
+
+  g_return_val_if_fail (error == NULL || *error == NULL, NULL);
+  g_return_val_if_fail (argv != NULL &&
+                        argv[0] != NULL &&
+                        argv[0][0] != '\0',
+                        NULL);
+
+  if (client->launcher == NULL)
+    {
+      g_set_error (error,
+                   G_IO_ERROR,
+                   G_IO_ERROR_NOT_INITIALIZED,
+                   "MetaWaylandClient must be created using meta_wayland_client_new().");
+      return NULL;
+    }
+
+  if (client->subprocess != NULL)
+    {
+      g_set_error (error,
+                   G_IO_ERROR,
+                   G_IO_ERROR_FAILED,
+                   "This object already has a process running.");
+      return NULL;
+    }
+
+  if (socketpair (AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0, client_fd) < 0)
+    {
+      g_set_error (error,
+                   G_IO_ERROR,
+                   G_IO_ERROR_FAILED,
+                   "Failed to create a socket pair for the wayland client.");
+      return NULL;
+    }
+
+  compositor = meta_wayland_compositor_get_default ();
+  g_subprocess_launcher_take_fd (client->launcher, client_fd[1], 3);
+  g_subprocess_launcher_setenv (client->launcher, "WAYLAND_SOCKET", "3", TRUE);
+  wayland_client = wl_client_create (compositor->wayland_display, client_fd[0]);
+  subprocess = g_subprocess_launcher_spawnv (client->launcher, argv, error);
+
+  if (subprocess == NULL)
+    return NULL;
+
+  client->subprocess = subprocess;
+  client->wayland_client = wayland_client;
+  client->process_running = TRUE;
+  client->died_cancellable = g_cancellable_new ();
+  g_subprocess_wait_async (client->subprocess,
+                           client->died_cancellable,
+                           process_died,
+                           client);
+
+  return g_object_ref (client->subprocess);
+}
+
+/**
+ * meta_wayland_client_spawn:
+ * @client: a #MetaWaylandClient
+ * @display: (not nullable): the current MetaDisplay
+ * @error: (nullable): Error
+ * @argv0: Command line arguments
+ * @...: Continued arguments, %NULL terminated
+ *
+ * Creates a #GSubprocess given a provided varargs list of arguments. It also
+ * sets up a new Wayland socket and sets the environment variable WAYLAND_SOCKET
+ * to make the new process to use it.
+ *
+ * Returns: (transfer full): A new #GSubprocess, or %NULL on error (and @error
+ * will be set)
+ **/
+GSubprocess *
+meta_wayland_client_spawn (MetaWaylandClient  *client,
+                           MetaDisplay        *display,
+                           GError            **error,
+                           const char         *argv0,
+                           ...)
+{
+  g_autoptr (GPtrArray) args = NULL;
+  GSubprocess *result;
+  const char *arg;
+  va_list ap;
+
+  g_return_val_if_fail (argv0 != NULL && argv0[0] != '\0', NULL);
+  g_return_val_if_fail (error == NULL || *error == NULL, NULL);
+
+  args = g_ptr_array_new_with_free_func (g_free);
+
+  va_start (ap, argv0);
+  g_ptr_array_add (args, (char *) argv0);
+  while ((arg = va_arg (ap, const char *)))
+    g_ptr_array_add (args, (char *) arg);
+
+  g_ptr_array_add (args, NULL);
+  va_end (ap);
+
+  result = meta_wayland_client_spawnv (client,
+                                       display,
+                                       (const char * const *) args->pdata,
+                                       error);
+
+  return result;
+}
+
+/**
+ * meta_wayland_client_owns_wayland_window
+ * @client: a #MetaWaylandClient
+ * @window: (not nullable): a MetaWindow
+ *
+ * Checks whether @window belongs to the process launched from @client or not.
+ * This only works under Wayland. If the window is an X11 window, an exception
+ * will be triggered.
+ *
+ * Returns: TRUE if the window was created by this process; FALSE if not.
+ */
+gboolean
+meta_wayland_client_owns_window (MetaWaylandClient *client,
+                                 MetaWindow        *window)
+{
+  MetaWaylandSurface *surface;
+
+  g_return_val_if_fail (meta_is_wayland_compositor (), FALSE);
+  g_return_val_if_fail (client->subprocess != NULL, FALSE);
+  g_return_val_if_fail (client->process_running, FALSE);
+
+  surface = window->surface;
+  if (surface == NULL)
+    return FALSE;
+
+  return wl_resource_get_client (surface->resource) == client->wayland_client;
+}
+
+/**
+ * meta_wayland_client_skip_from_window_list
+ * @client: a #MetaWaylandClient
+ * @window: (not nullable): a MetaWindow
+ *
+ * Hides this window from any window list, like taskbars, pagers...
+ */
+void
+meta_wayland_client_hide_from_window_list (MetaWaylandClient *client,
+                                           MetaWindow        *window)
+{
+  if (!meta_wayland_client_owns_window (client, window))
+    return;
+
+  if (!window->skip_from_window_list)
+    {
+      window->skip_from_window_list = TRUE;
+      meta_window_recalc_features (window);
+    }
+}
+
+/**
+ * meta_wayland_client_show_in_window_list
+ * @client: a #MetaWaylandClient
+ * @window: (not nullable): a MetaWindow
+ *
+ * Shows again this window in window lists, like taskbars, pagers...
+ */
+void
+meta_wayland_client_show_in_window_list (MetaWaylandClient *client,
+                                         MetaWindow        *window)
+{
+  if (!meta_wayland_client_owns_window (client, window))
+    return;
+
+  if (window->skip_from_window_list)
+    {
+      window->skip_from_window_list = FALSE;
+      meta_window_recalc_features (window);
+    }
+}