gh-92112: Fix crash triggered by an evil custom mro() (#92113)

Lib/test/test_descr.py

@@ -5784,6 +5784,23 @@ def mro(cls):
         class A(metaclass=M):
             pass
 
+    def test_disappearing_custom_mro(self):
+        """
+        gh-92112: A custom mro() returning a result conflicting with
+        __bases__ and deleting itself caused a double free.
+        """
+        class B:
+            pass
+
+        class M(DebugHelperMeta):
+            def mro(cls):
+                del M.mro
+                return (B,)
+
+        with self.assertRaises(TypeError):
+            class A(metaclass=M):
+                pass
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2022-05-01-10-58-38.gh-issue-92112.lLJemu.rst

@@ -0,0 +1 @@
+Fix crash triggered by an evil custom ``mro()`` on a metaclass.

Objects/typeobject.c

@@ -345,22 +345,26 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
     Py_ssize_t i, n;
     int custom = !Py_IS_TYPE(type, &PyType_Type);
     int unbound;
-    PyObject *mro_meth = NULL;
-    PyObject *type_mro_meth = NULL;
 
     if (custom) {
+        PyObject *mro_meth, *type_mro_meth;
         mro_meth = lookup_maybe_method(
             (PyObject *)type, &_Py_ID(mro), &unbound);
-        if (mro_meth == NULL)
+        if (mro_meth == NULL) {
             goto clear;
+        }
         type_mro_meth = lookup_maybe_method(
             (PyObject *)&PyType_Type, &_Py_ID(mro), &unbound);
-        if (type_mro_meth == NULL)
+        if (type_mro_meth == NULL) {
+            Py_DECREF(mro_meth);
             goto clear;
-        if (mro_meth != type_mro_meth)
+        }
+        int custom_mro = (mro_meth != type_mro_meth);
+        Py_DECREF(mro_meth);
+        Py_DECREF(type_mro_meth);
+        if (custom_mro) {
             goto clear;
-        Py_XDECREF(mro_meth);
-        Py_XDECREF(type_mro_meth);
+        }
     }
     n = PyTuple_GET_SIZE(bases);
     for (i = 0; i < n; i++) {
@@ -373,8 +377,6 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
     }
     return;
  clear:
-    Py_XDECREF(mro_meth);
-    Py_XDECREF(type_mro_meth);
     type->tp_flags &= ~Py_TPFLAGS_VALID_VERSION_TAG;
     type->tp_version_tag = 0; /* 0 is not a valid version tag */
 }