Fix phpGH-20352: UAF in php_output_handler_free via re-entrant ob_start() during error deactivation

The problem is that the code is doing `php_output_handler_free` in a loop on the output stack,
but prior to freeing the pointer on the stack in `php_output_handler_free` it calls
`php_output_handler_dtor` which can run user code that reallocates the stack,
resulting in a dangling pointer freed by php_output_handler_free.
Furthermore, OG(active) is set when creating a new output handler, but
the loop is supposed to clean up all handlers, so OG(active) must be
reset as well.

Closes phpGH-20356.

Author: ndossche

NEWS

@@ -8,6 +8,8 @@ PHP                                                                        NEWS
   . Fixed bug GH-20695 (Assertion failure in normalize_value() when parsing
     malformed INI input via parse_ini_string()). (ndossche)
   . Fixed bug GH-20714 (Uncatchable exception thrown in generator). (ilutov)
+  . Fixed bug GH-20352 (UAF in php_output_handler_free via re-entrant
+    ob_start() during error deactivation). (ndossche)
 
 - Bz2:
   . Fixed bug GH-20620 (bzcompress overflow on large source size).

main/output.c

@@ -188,8 +188,12 @@ PHPAPI void php_output_deactivate(void)
 		/* release all output handlers */
 		if (OG(handlers).elements) {
 			while ((handler = zend_stack_top(&OG(handlers)))) {
-				php_output_handler_free(handler);
 				zend_stack_del_top(&OG(handlers));
+				/* It's possible to start a new output handler and mark it as active,
+				 * however this loop will destroy all active handlers. */
+				OG(active) = NULL;
+				ZEND_ASSERT(OG(running) == NULL && "output is deactivated therefore running should stay NULL");
+				php_output_handler_free(handler);
 			}
 		}
 		zend_stack_destroy(&OG(handlers));
@@ -719,10 +723,11 @@ PHPAPI void php_output_handler_dtor(php_output_handler *handler)
  * Destroy and free an output handler */
 PHPAPI void php_output_handler_free(php_output_handler **h)
 {
-	if (*h) {
-		php_output_handler_dtor(*h);
-		efree(*h);
+	php_output_handler *handler = *h;
+	if (handler) {
 		*h = NULL;
+		php_output_handler_dtor(handler);
+		efree(handler);
 	}
 }
 /* }}} */

tests/output/gh20352.phpt

@@ -0,0 +1,24 @@
+--TEST--
+GH-20352 (UAF in php_output_handler_free via re-entrant ob_start() during error deactivation)
+--FILE--
+<?php
+class Test {
+    public function __destruct() {
+        // Spray output stack
+        for ($i = 0; $i < 1000; $i++)
+            ob_start(static function() {});
+    }
+
+    public function __invoke($x) {
+        // Trigger php_output_deactivate() through forbidden operation
+        ob_start('foo');
+        return $x;
+    }
+}
+
+ob_start(new Test, 1);
+
+echo "trigger bug";
+?>
+--EXPECTF--
+Fatal error: ob_start(): Cannot use output buffering in output buffering display handlers in %s on line %d