Commit message: dir: Refuse to export .desktop files with suspicious uses of @@ tokens

This is either a malicious/compromised app trying to do an attack, or
a mistake that will break handling of %f, %u and so on. Either way,
if we refuse to export the .desktop file, resulting in installation
failing, then it makes the rejection more obvious than quietly
removing the magic tokens.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 46b3ede5241561c7d588951048c687c5075a3eac)
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 if (g_str_has_prefix (arg, "@@"))
                {
                  flatpak_fail_error (error, FLATPAK_ERROR_EXPORT_FAILED,
                                     _("Invalid Exec argument %s"), arg);
                  goto out;
                }
              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;
}