MDL-68486 auth_shibboleth: Prevent using dataroot files in convert_data

Prevents configuring the 'Data modification API' (convert_data) setting
to use files located within the $CFG->dataroot directory as it exposes
the site to security risks.

auth/shibboleth/classes/admin_setting_special_convert_data_configfile.php

@@ -0,0 +1,75 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle 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 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle 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 Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Special setting for auth_shibboleth convert_data.
+ *
+ * @package    auth_shibboleth
+ * @copyright  2020 Mihail Geshoski
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+/**
+ * Admin settings class for the convert_data option.
+ *
+ * @package    auth_shibboleth
+ * @copyright  2020 Mihail Geshoski
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class auth_shibboleth_admin_setting_convert_data extends admin_setting_configfile {
+
+    /**
+     * Constructor.
+     *
+     * @param string $name
+     * @param string $visiblename
+     * @param string $description
+     * @param mixed $defaultdirectory
+     */
+    public function __construct($name, $visiblename, $description, $defaultdirectory) {
+        parent::__construct($name, $visiblename, $description, $defaultdirectory);
+    }
+
+    /**
+     * Validate the file path (location).
+     *
+     * This method ensures that the file defined as a data modification API exists and is not located in the site
+     * data directory ($CFG->dataroot). We should prohibit using files from the site data directory as this introduces
+     * security vulnerabilities.
+     *
+     * @param string $filepath The path to the file.
+     * @return mixed bool true for success or string:error on failure.
+     */
+    public function validate($filepath) {
+        global $CFG;
+
+        if (empty($filepath)) {
+            return true;
+        }
+
+        // Fail if the file does not exist or it is not readable by the webserver process.
+        if (!is_readable($filepath)) {
+            return get_string('auth_shib_convert_data_warning', 'auth_shibboleth');
+        }
+
+        // Fail if the absolute file path matches the currently defined dataroot path.
+        if (preg_match('/' . preg_quote($CFG->dataroot, '/') . '/', realpath($filepath))) {
+            return get_string('auth_shib_convert_data_filepath_warning', 'auth_shibboleth');
+        }
+
+        return true;
+    }
+}