bpo-44856: Possible reference leak in error paths of update_bases() and __build_class__ (GH-27647)

Author: pablogsal

Misc/NEWS.d/next/Core and Builtins/2021-08-07-01-26-12.bpo-44856.9rk3li.rst

@@ -0,0 +1 @@
+Fix reference leaks in the error paths of ``update_bases()`` and ``__build_class__``. Patch by Pablo Galindo.

Python/bltinmodule.c

@@ -72,6 +72,7 @@ update_bases(PyObject *bases, PyObject *const *args, Py_ssize_t nargs)
             /* If this is a first successful replacement, create new_bases list and
                copy previously encountered bases. */
             if (!(new_bases = PyList_New(i))) {
+                Py_DECREF(new_base);
                 goto error;
             }
             for (j = 0; j < i; j++) {
@@ -82,6 +83,7 @@ update_bases(PyObject *bases, PyObject *const *args, Py_ssize_t nargs)
         }
         j = PyList_GET_SIZE(new_bases);
         if (PyList_SetSlice(new_bases, j, j, new_base) < 0) {
+            Py_DECREF(new_base);
             goto error;
         }
         Py_DECREF(new_base);
@@ -103,8 +105,9 @@ static PyObject *
 builtin___build_class__(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
                         PyObject *kwnames)
 {
-    PyObject *func, *name, *bases, *mkw, *meta, *winner, *prep, *ns, *orig_bases;
-    PyObject *cls = NULL, *cell = NULL;
+    PyObject *func, *name, *winner, *prep;
+    PyObject *cls = NULL, *cell = NULL, *ns = NULL, *meta = NULL, *orig_bases = NULL;
+    PyObject *mkw = NULL, *bases = NULL;
     int isclass = 0;   /* initialize to prevent gcc warning */
 
     if (nargs < 2) {
@@ -141,26 +144,20 @@ builtin___build_class__(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
     else {
         mkw = _PyStack_AsDict(args + nargs, kwnames);
         if (mkw == NULL) {
-            Py_DECREF(bases);
-            return NULL;
+            goto error;
         }
 
         meta = _PyDict_GetItemIdWithError(mkw, &PyId_metaclass);
         if (meta != NULL) {
             Py_INCREF(meta);
             if (_PyDict_DelItemId(mkw, &PyId_metaclass) < 0) {
-                Py_DECREF(meta);
-                Py_DECREF(mkw);
-                Py_DECREF(bases);
-                return NULL;
+                goto error;
             }
             /* metaclass is explicitly given, check if it's indeed a class */
             isclass = PyType_Check(meta);
         }
         else if (PyErr_Occurred()) {
-            Py_DECREF(mkw);
-            Py_DECREF(bases);
-            return NULL;
+            goto error;
         }
     }
     if (meta == NULL) {
@@ -183,10 +180,7 @@ builtin___build_class__(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
         winner = (PyObject *)_PyType_CalculateMetaclass((PyTypeObject *)meta,
                                                         bases);
         if (winner == NULL) {
-            Py_DECREF(meta);
-            Py_XDECREF(mkw);
-            Py_DECREF(bases);
-            return NULL;
+            goto error;
         }
         if (winner != meta) {
             Py_DECREF(meta);
@@ -208,10 +202,7 @@ builtin___build_class__(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
         Py_DECREF(prep);
     }
     if (ns == NULL) {
-        Py_DECREF(meta);
-        Py_XDECREF(mkw);
-        Py_DECREF(bases);
-        return NULL;
+        goto error;
     }
     if (!PyMapping_Check(ns)) {
         PyErr_Format(PyExc_TypeError,
@@ -252,13 +243,13 @@ builtin___build_class__(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
     }
 error:
     Py_XDECREF(cell);
-    Py_DECREF(ns);
-    Py_DECREF(meta);
+    Py_XDECREF(ns);
+    Py_XDECREF(meta);
     Py_XDECREF(mkw);
-    Py_DECREF(bases);
     if (bases != orig_bases) {
         Py_DECREF(orig_bases);
     }
+    Py_DECREF(bases);
     return cls;
 }