MDL-67175 session: set SameSite=None for Chrome 78 and above

Totara reference TL-22311 (original code by Brendan Cox and Sam Hemelryk)
a3f4de2

lib/classes/session/manager.php

@@ -260,7 +260,27 @@ protected static function prepare_cookies() {
 
         // Set configuration.
         session_name($sessionname);
-        session_set_cookie_params(0, $CFG->sessioncookiepath, $CFG->sessioncookiedomain, $cookiesecure, $CFG->cookiehttponly);
+
+        if (version_compare(PHP_VERSION, '7.3.0', '>=')) {
+            $sessionoptions = [
+                'lifetime' => 0,
+                'path' => $CFG->sessioncookiepath,
+                'domain' => $CFG->sessioncookiedomain,
+                'secure' => $cookiesecure,
+                'httponly' => $CFG->cookiehttponly,
+            ];
+
+            if (self::should_use_samesite_none()) {
+                // If $samesite is empty, we don't want there to be any SameSite attribute.
+                $sessionoptions['samesite'] = 'None';
+            }
+
+            session_set_cookie_params($sessionoptions);
+        } else {
+            // Once PHP 7.3 becomes our minimum, drop this in favour of the alternative call to session_set_cookie_params above,
+            // as that does not require a hack to work with same site settings on cookies.
+            session_set_cookie_params(0, $CFG->sessioncookiepath, $CFG->sessioncookiedomain, $cookiesecure, $CFG->cookiehttponly);
+        }
         ini_set('session.use_trans_sid', '0');
         ini_set('session.use_only_cookies', '1');
         ini_set('session.hash_function', '0');        // For now MD5 - we do not have room for sha-1 in sessions table.
@@ -420,6 +440,8 @@ protected static function initialise_user_session($newsid) {
         if ($timedout) {
             $_SESSION['SESSION']->has_timed_out = true;
         }
+
+        self::append_samesite_cookie_attribute();
     }
 
     /**
@@ -487,6 +509,61 @@ public static function login_user(\stdClass $user) {
 
         // Setup $USER object.
         self::set_user($user);
+        self::append_samesite_cookie_attribute();
+    }
+
+    /**
+     * Returns a valid setting for the SameSite cookie attribute.
+     *
+     * @return string The desired setting for the SameSite attribute on the cookie. Empty string indicates the SameSite attribute
+     * should not be set at all.
+     */
+    private static function should_use_samesite_none() {
+        // We only want None or no attribute at this point. When we have cookie handling compatible with Lax,
+        // we can look at checking a setting.
+
+        // Browser support for none is not consistent yet. There are known issues with Safari, and IE11.
+        // Things are stablising, however as they're not stable yet we will deal specifically with the version of chrome
+        // that introduces a default of lax, setting it to none for the current version of chrome (2 releases before the change).
+        // We also check you are using secure cookies and HTTPS because if you are not running over HTTPS
+        // then setting SameSite=None will cause your session cookie to be rejected.
+        if (\core_useragent::is_chrome() && \core_useragent::check_chrome_version('78') && is_moodle_cookie_secure()) {
+            return true;
+        }
+        return false;
+    }
+
+    /**
+     * Conditionally append the SameSite attribute to the session cookie if necessary.
+     *
+     * Contains a hack for versions of PHP lower than 7.3 as there is no API built into PHP cookie API
+     * for adding the SameSite setting.
+     *
+     * This won't change the Set-Cookie headers if:
+     *  - PHP 7.3 or higher is being used. That already adds the SameSite attribute without any hacks.
+     *  - If the samesite setting is empty.
+     *  - If the samesite setting is None but the browser is not compatible with that setting.
+     */
+    private static function append_samesite_cookie_attribute() {
+        if (version_compare(PHP_VERSION, '7.3.0', '>=')) {
+            // This hack is only necessary if we weren't able to set the samesite flag via the session_set_cookie_params API.
+            return;
+        }
+
+        if (!self::should_use_samesite_none()) {
+            return;
+        }
+
+        $cookies = headers_list();
+        header_remove('Set-Cookie');
+        $setcookiesession = 'Set-Cookie: ' . session_name() . '=';
+
+        foreach ($cookies as $cookie) {
+            if (strpos($cookie, $setcookiesession) === 0) {
+                $cookie .= '; SameSite=None';
+            }
+            header($cookie, false);
+        }
     }
 
     /**
@@ -523,6 +600,8 @@ public static function terminate_current() {
         self::add_session_record($_SESSION['USER']->id); // Do not use $USER here because it may not be set up yet.
         session_write_close();
         self::$sessionactive = false;
+
+        self::append_samesite_cookie_attribute();
     }
 
     /**