MDL-71887 mod_lti: repost when no cookie due to crosssite request

mod/lti/auth.php

@@ -24,6 +24,19 @@
 
 require_once(__DIR__ . '/../../config.php');
 require_once($CFG->dirroot . '/mod/lti/locallib.php');
+global $_POST, $_SERVER;
+
+if (!isloggedin() && empty($_POST['repost'])) {
+    header_remove("Set-Cookie");
+    $PAGE->set_pagelayout('popup');
+    $PAGE->set_context(context_system::instance());
+    $output = $PAGE->get_renderer('mod_lti');
+    $page = new \mod_lti\output\repost_crosssite_page($_SERVER['REQUEST_URI'], $_POST);
+    echo $output->header();
+    echo $output->render($page);
+    echo $output->footer();
+    return;
+}
 
 $scope = optional_param('scope', '', PARAM_TEXT);
 $responsetype = optional_param('response_type', '', PARAM_TEXT);

mod/lti/classes/output/renderer.php

@@ -71,4 +71,17 @@ class renderer extends plugin_renderer_base {
         $data = $page->export_for_template($this);
         return parent::render_from_template('mod_lti/registration_upgrade_choice_page', $data);
     }
+
+    /**
+     * Render the reposting of the cross site request.
+     *
+     * @param repost_crosssite_page $page the page renderable.
+     *
+     * @return string rendered html for the page.
+     */
+    public function render_repost_crosssite_page(repost_crosssite_page $page): string {
+        $data = $page->export_for_template($this);
+        return parent::render_from_template('mod_lti/repost_crosssite', $data);
+    }
+
 }

mod/lti/classes/output/repost_crosssite_page.php

@@ -0,0 +1,76 @@
+<?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/>.
+
+/**
+ * Render a page containing a simple form which reposts to self via JS.
+ *
+ * The purpose of this form is to resend a cross-site request to self, which allows the browsers to include the Moodle
+ * session cookie alongside the original POST data, allowing LTI flows to function despite browsers blocking
+ * cross-site cookies.
+ *
+ * @copyright  2021 Cengage
+ * @package    mod_lti
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+namespace mod_lti\output;
+
+defined('MOODLE_INTERNAL') || die;
+
+require_once($CFG->dirroot.'/mod/lti/locallib.php');
+
+use renderable;
+use templatable;
+use renderer_base;
+use stdClass;
+
+/**
+ * Render a page containing a simple form which reposts to self via JS.
+ *
+ * The purpose of this form is to resend a cross-site request to self, which allows the browsers to include the Moodle
+ * session cookie alongside the original POST data, allowing LTI flows to function despite browsers blocking
+ * cross-site cookies.
+ *
+ * @copyright  2021 Cengage
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class repost_crosssite_page implements renderable, templatable {
+
+    /**
+     * Constructor
+     *
+     * @param string $url moodle URL to repost to
+     * @param array $post the POST params to be re-posted
+     */
+    public function __construct(string $url, array $post) {
+        $this->params = array_map(function($k) use ($post) {
+            return ["key" => $k, "value" => str_replace("\"", "&quot;", $post[$k])];
+        }, array_keys($post));
+        $this->url = $url;
+    }
+
+    /**
+     * Export this data so it can be used as the context for a mustache template.
+     *
+     * @param renderer_base $output The renderer
+     * @return stdClass Data to be used by the template
+     */
+    public function export_for_template(renderer_base $output) {
+        $renderdata = new stdClass();
+        $renderdata->url = $this->url;
+        $renderdata->params = $this->params;
+        return $renderdata;
+    }
+}

mod/lti/contentitem_return.php

@@ -31,6 +31,28 @@ $courseid = required_param('course', PARAM_INT);
 
 $jwt = optional_param('JWT', '', PARAM_RAW);
 
+$context = context_course::instance($courseid);
+
+$pageurl = new moodle_url('/mod/lti/contentitem_return.php');
+$PAGE->set_url($pageurl);
+$PAGE->set_pagelayout('popup');
+$PAGE->set_context($context);
+
+// Cross-Site causes the cookie to be lost if not POSTed from same site.
+global $_POST;
+if (!empty($_POST["repost"])) {
+    // Unset the param so that LTI 1.1 signature validation passes.
+    unset($_POST["repost"]);
+} else if (!isloggedin()) {
+    header_remove("Set-Cookie");
+    $output = $PAGE->get_renderer('mod_lti');
+    $page = new \mod_lti\output\repost_crosssite_page($_SERVER['REQUEST_URI'], $_POST);
+    echo $output->header();
+    echo $output->render($page);
+    echo $output->footer();
+    return;
+}
+
 if (!empty($jwt)) {
     $params = lti_convert_from_jwt($id, $jwt);
     $consumerkey = $params['oauth_consumer_key'] ?? '';
@@ -52,7 +74,6 @@ if (!empty($jwt)) {
 $course = $DB->get_record('course', array('id' => $courseid), '*', MUST_EXIST);
 require_login($course);
 require_sesskey();
-$context = context_course::instance($courseid);
 require_capability('moodle/course:manageactivities', $context);
 require_capability('mod/lti:addcoursetool', $context);
 
@@ -66,9 +87,6 @@ if (empty($errormsg) && !empty($items)) {
     }
 }
 
-$pageurl = new moodle_url('/mod/lti/contentitem_return.php');
-$PAGE->set_url($pageurl);
-$PAGE->set_pagelayout('popup');
 echo $OUTPUT->header();
 
 // Call JS module to redirect the user to the course page or close the dialogue on error/cancel.

mod/lti/templates/repost_crosssite.mustache

@@ -0,0 +1,49 @@
+{{!
+    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/>.
+}}
+{{!
+    @template mod_lti/repost_crosssite
+
+    This template re-executes a cross site request but from the site domain, avoiding the cross site issue.
+
+    Classes required for JS:
+    * none
+
+    Data attributes required for JS:
+    * none
+
+    Context variables required for this template:
+    *
+
+    Example context (json):
+    {
+        "url":"https://some.tool.example/mod/lti/auth.php",
+        "params": {
+            "response_type": "id_token"
+        }
+    }
+
+}}
+<form action="{{{url}}}" method="POST" id="autopostme">
+    {{#params}}
+    <input type="hidden" name="{{{key}}}" value="{{{value}}}">
+    {{/params}}
+    <input type="hidden" name="repost" value="true">
+</form>
+
+{{#js}}
+        document.getElementById("autopostme").submit();
+{{/js}}