Protect against stack overflow if the user derives from Message.

php/ext/google/protobuf/message.c

@@ -554,10 +554,32 @@ static void Message_Initialize(Message *intern, const Descriptor *desc) {
  */
 PHP_METHOD(Message, __construct) {
   Message* intern = (Message*)Z_OBJ_P(getThis());
-  const Descriptor* desc = Descriptor_GetFromClassEntry(Z_OBJCE_P(getThis()));
+  const Descriptor* desc;
+  zend_class_entry *ce = Z_OBJCE_P(getThis());
   upb_arena *arena = Arena_Get(&intern->arena);
   zval *init_arr = NULL;
 
+  // This descriptor should always be available, as the generated __construct
+  // method calls initOnce() to load the descriptor prior to calling us.
+  //
+  // However, if the user created their own class derived from Message, this
+  // will trigger an infinite construction loop and blow the stack.  We
+  // temporarily clear create_object to break this loop (see check in
+  // NameMap_GetMessage()).
+  PBPHP_ASSERT(ce->create_object == Message_create);
+  ce->create_object = NULL;
+  desc = Descriptor_GetFromClassEntry(ce);
+  ce->create_object = Message_create;
+
+  if (!desc) {
+    zend_throw_exception_ex(
+        NULL, 0,
+        "Couldn't find descriptor. Note only generated code may derive from "
+        "\\Google\\Protobuf\\Internal\\Message");
+    return;
+  }
+
+
   Message_Initialize(intern, desc);
 
   if (zend_parse_parameters(ZEND_NUM_ARGS(), "|a!", &init_arr) == FAILURE) {

php/src/Google/Protobuf/Internal/Message.php

@@ -93,8 +93,9 @@ private function initWithGeneratedPool()
         $pool = DescriptorPool::getGeneratedPool();
         $this->desc = $pool->getDescriptorByClassName(get_class($this));
         if (is_null($this->desc)) {
-            user_error(get_class($this) . " is not found in descriptor pool.");
-            return;
+          throw new \InvalidArgumentException(
+            get_class($this) ." is not found in descriptor pool. " .
+            'Only generated classes may derive from Message.');
         }
         foreach ($this->desc->getField() as $field) {
             $setter = $field->getSetter();

php/tests/GeneratedClassTest.php

@@ -24,6 +24,13 @@
 use PBEmpty\PBEcho\TestEmptyPackage;
 use Php\Test\TestNamespace;
 
+# This is not allowed, but we at least shouldn't crash.
+class C extends \Google\Protobuf\Internal\Message {
+    public function __construct($data = null) {
+        parent::__construct($data);
+    }
+}
+
 class GeneratedClassTest extends TestBase
 {
 
@@ -1723,6 +1730,16 @@ public function testHasOneof() {
         $this->assertFalse($m->hasOneofString());
     }
 
+    #########################################################
+    # Test that we don't crash if users create their own messages.
+    #########################################################
+
+    public function testUserDefinedClass() {
+      # This is not allowed, but at least we shouldn't crash.
+      $this->expectException(Exception::class);
+      $p = new C();
+    }
+
     #########################################################
     # Test no segfault when error happens
     #########################################################