Commit message: Disallow @@ and @@u usage in desktop files

Fixes #4146.

(cherry picked from commit 652a28ffab67cb6cd7d12dc3a93979bcd3731c7f)
CVE: CVE-2021-21381
CWE: CWE-94
Vulnerability description: Flatpak is a system for building, distributing, and running sandboxed desktop applications on Linux. In Flatpack since version 0.9.4 and before version 1.10.2 has a vulnerability in the "file forwarding" feature which can be used by an attacker to gain access to files that would not ordinarily be allowed by the app's permissions. By putting the special tokens `@@` and/or `@@u` in the Exec field of a Flatpak app's .desktop file, a malicious app publisher can trick flatpak into behaving as though the user had chosen to open a target file with their Flatpak app, which automatically makes that file available to the Flatpak app. This is fixed in version 1.10.2. A minimal solution is the first commit "`Disallow @@ and @@U usage in desktop files`". The follow-up commits "`dir: Reserve the whole @@ prefix`" and "`dir: Refuse to export .desktop files with suspicious uses of @@ tokens`" are recommended, but not strictly required. As a workaround, avoid installing Flatpak apps from untrusted sources, or check the contents of the exported `.desktop` files in `exports/share/applications/*.desktop` (typically `~/.local/share/flatpak/exports/share/applications/*.desktop` and `/var/lib/flatpak/exports/share/applications/*.desktop`) to make sure that literal filenames do not follow `@@` or `@@u`.

Project: flatpak
File: flatpak-dir.c

export_desktop_file (const char         *app,
                     const char         *branch,
                     const char         *arch,
                     GKeyFile           *metadata,
                     const char * const *previous_ids,
                     int                 parent_fd,
                     const char         *name,
                     struct stat        *stat_buf,
                     char              **target,
                     GCancellable       *cancellable,
                     GError            **error)
{
  gboolean ret = FALSE;
  glnx_autofd int desktop_fd = -1;
  g_autofree char *tmpfile_name = g_strdup_printf ("export-desktop-XXXXXX");
  g_autoptr(GOutputStream) out_stream = NULL;
  g_autofree gchar *data = NULL;
  gsize data_len;
  g_autofree gchar *new_data = NULL;
  gsize new_data_len;
  g_autoptr(GKeyFile) keyfile = NULL;
  g_autofree gchar *old_exec = NULL;
  gint old_argc;
  g_auto(GStrv) old_argv = NULL;
  g_auto(GStrv) groups = NULL;
  GString *new_exec = NULL;
  g_autofree char *escaped_app = maybe_quote (app);
  g_autofree char *escaped_branch = maybe_quote (branch);
  g_autofree char *escaped_arch = maybe_quote (arch);
  int i;

  if (!flatpak_openat_noatime (parent_fd, name, &desktop_fd, cancellable, error))
    goto out;

  if (!read_fd (desktop_fd, stat_buf, &data, &data_len, error))
    goto out;

  keyfile = g_key_file_new ();
  if (!g_key_file_load_from_data (keyfile, data, data_len, G_KEY_FILE_KEEP_TRANSLATIONS, error))
    goto out;

  if (g_str_has_suffix (name, ".service"))
    {
      g_autofree gchar *dbus_name = NULL;
      g_autofree gchar *expected_dbus_name = g_strndup (name, strlen (name) - strlen (".service"));

      dbus_name = g_key_file_get_string (keyfile, "D-BUS Service", "Name", NULL);

      if (dbus_name == NULL || strcmp (dbus_name, expected_dbus_name) != 0)
        {
          return flatpak_fail_error (error, FLATPAK_ERROR_EXPORT_FAILED,
                                     _("D-Bus service file '%s' has wrong name"), name);
        }
    }

  if (g_str_has_suffix (name, ".desktop"))
    {
      gsize length;
      g_auto(GStrv) tags = g_key_file_get_string_list (metadata,
                                                       "Application",
                                                       "tags", &length,
                                                       NULL);

      if (tags != NULL)
        {
          g_key_file_set_string_list (keyfile,
                                      G_KEY_FILE_DESKTOP_GROUP,
                                      "X-Flatpak-Tags",
                                      (const char * const *) tags, length);
        }

      /* Add a marker so consumers can easily find out that this launches a sandbox */
      g_key_file_set_string (keyfile, G_KEY_FILE_DESKTOP_GROUP, "X-Flatpak", app);

      /* If the app has been renamed, add its old .desktop filename to
       * X-Flatpak-RenamedFrom in the new .desktop file, taking care not to
       * introduce duplicates.
       */
      if (previous_ids != NULL)
        {
          const char *X_FLATPAK_RENAMED_FROM = "X-Flatpak-RenamedFrom";
          g_auto(GStrv) renamed_from = g_key_file_get_string_list (keyfile,
                                                                   G_KEY_FILE_DESKTOP_GROUP,
                                                                   X_FLATPAK_RENAMED_FROM,
                                                                   NULL, NULL);
          g_autoptr(GPtrArray) merged = g_ptr_array_new_with_free_func (g_free);
          g_autoptr(GHashTable) seen = g_hash_table_new (g_str_hash, g_str_equal);
          const char *new_suffix;

          for (i = 0; renamed_from != NULL && renamed_from[i] != NULL; i++)
            {
              if (!g_hash_table_contains (seen, renamed_from[i]))
                {
                  gchar *copy = g_strdup (renamed_from[i]);
                  g_hash_table_insert (seen, copy, copy);
                  g_ptr_array_add (merged, g_steal_pointer (&copy));
                }
            }

          /* If an app was renamed from com.example.Foo to net.example.Bar, and
           * the new version exports net.example.Bar-suffix.desktop, we assume the
           * old version exported com.example.Foo-suffix.desktop.
           *
           * This assertion is true because
           * flatpak_name_matches_one_wildcard_prefix() is called on all
           * exported files before we get here.
           */
          g_assert (g_str_has_prefix (name, app));
          /* ".desktop" for the "main" desktop file; something like
           * "-suffix.desktop" for extra ones.
           */
          new_suffix = name + strlen (app);

          for (i = 0; previous_ids[i] != NULL; i++)
            {
              g_autofree gchar *previous_desktop = g_strconcat (previous_ids[i], new_suffix, NULL);
              if (!g_hash_table_contains (seen, previous_desktop))
                {
                  g_hash_table_insert (seen, previous_desktop, previous_desktop);
                  g_ptr_array_add (merged, g_steal_pointer (&previous_desktop));
                }
            }

          if (merged->len > 0)
            {
              g_ptr_array_add (merged, NULL);
              g_key_file_set_string_list (keyfile,
                                          G_KEY_FILE_DESKTOP_GROUP,
                                          X_FLATPAK_RENAMED_FROM,
                                          (const char * const *) merged->pdata,
                                          merged->len - 1);
            }
        }
    }

  groups = g_key_file_get_groups (keyfile, NULL);

  for (i = 0; groups[i] != NULL; i++)
    {
      g_auto(GStrv) flatpak_run_opts = g_key_file_get_string_list (keyfile, groups[i], "X-Flatpak-RunOptions", NULL, NULL);
      g_autofree char *flatpak_run_args = format_flatpak_run_args_from_run_opts (flatpak_run_opts);

      g_key_file_remove_key (keyfile, groups[i], "X-Flatpak-RunOptions", NULL);
      g_key_file_remove_key (keyfile, groups[i], "TryExec", NULL);

      /* Remove this to make sure nothing tries to execute it outside the sandbox*/
      g_key_file_remove_key (keyfile, groups[i], "X-GNOME-Bugzilla-ExtraInfoScript", NULL);

      new_exec = g_string_new ("");
      g_string_append_printf (new_exec,
                              FLATPAK_BINDIR "/flatpak run --branch=%s --arch=%s",
                              escaped_branch,
                              escaped_arch);

      if (flatpak_run_args != NULL)
        g_string_append_printf (new_exec, "%s", flatpak_run_args);

      old_exec = g_key_file_get_string (keyfile, groups[i], "Exec", NULL);
      if (old_exec && g_shell_parse_argv (old_exec, &old_argc, &old_argv, NULL) && old_argc >= 1)
        {
          int j;
          g_autofree char *command = maybe_quote (old_argv[0]);

          g_string_append_printf (new_exec, " --command=%s", command);

          for (j = 1; j < old_argc; j++)
            {
              if (strcasecmp (old_argv[j], "%f") == 0 ||
                  strcasecmp (old_argv[j], "%u") == 0)
                {
                  g_string_append (new_exec, " --file-forwarding");
                  break;
                }
            }

          g_string_append (new_exec, " ");
          g_string_append (new_exec, escaped_app);

          for (j = 1; j < old_argc; j++)
            {
              g_autofree char *arg = maybe_quote (old_argv[j]);

              if (strcasecmp (arg, "%f") == 0)
                g_string_append_printf (new_exec, " @@ %s @@", arg);
              else if (strcasecmp (arg, "%u") == 0)
                g_string_append_printf (new_exec, " @@u %s @@", arg);
              else
                g_string_append_printf (new_exec, " %s", arg);
            }
        }
      else
        {
          g_string_append (new_exec, " ");
          g_string_append (new_exec, escaped_app);
        }

      g_key_file_set_string (keyfile, groups[i], G_KEY_FILE_DESKTOP_KEY_EXEC, new_exec->str);
    }

  new_data = g_key_file_to_data (keyfile, &new_data_len, error);
  if (new_data == NULL)
    goto out;

  if (!flatpak_open_in_tmpdir_at (parent_fd, 0755, tmpfile_name, &out_stream, cancellable, error))
    goto out;

  if (!g_output_stream_write_all (out_stream, new_data, new_data_len, NULL, cancellable, error))
    goto out;

  if (!g_output_stream_close (out_stream, cancellable, error))
    goto out;

  if (target)
    *target = g_steal_pointer (&tmpfile_name);

  ret = TRUE;
out:

  if (new_exec != NULL)
    g_string_free (new_exec, TRUE);

  return ret;
}