[3.13] gh-142783: Fix possible use after free in zoneinfo module (GH-142790)

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -1551,6 +1551,26 @@ def __eq__(self, other):
         except CustomError:
             pass
 
+    def test_weak_cache_descriptor_use_after_free(self):
+        class BombDescriptor:
+            def __get__(self, obj, owner):
+                return {}
+
+        class EvilZoneInfo(self.klass):
+            pass
+
+        # Must be set after the class creation.
+        EvilZoneInfo._weak_cache = BombDescriptor()
+
+        key = "America/Los_Angeles"
+        zone1 = EvilZoneInfo(key)
+        self.assertEqual(str(zone1), key)
+
+        EvilZoneInfo.clear_cache()
+        zone2 = EvilZoneInfo(key)
+        self.assertEqual(str(zone2), key)
+        self.assertIsNot(zone2, zone1)
+
 
 class CZoneInfoCacheTest(ZoneInfoCacheTest):
     module = c_zoneinfo

Misc/NEWS.d/next/Library/2025-12-16-14-49-19.gh-issue-142783.VPV1ig.rst

@@ -0,0 +1 @@
+Fix zoneinfo use-after-free with descriptor _weak_cache. a descriptor as _weak_cache could cause crashes during object creation. The fix ensures proper reference counting for descriptor-provided objects.

Modules/_zoneinfo.c

@@ -287,16 +287,11 @@ static PyObject *
 get_weak_cache(zoneinfo_state *state, PyTypeObject *type)
 {
     if (type == state->ZoneInfoType) {
+        Py_INCREF(state->ZONEINFO_WEAK_CACHE);
         return state->ZONEINFO_WEAK_CACHE;
     }
     else {
-        PyObject *cache =
-            PyObject_GetAttrString((PyObject *)type, "_weak_cache");
-        // We are assuming that the type lives at least as long as the function
-        // that calls get_weak_cache, and that it holds a reference to the
-        // cache, so we'll return a "borrowed reference".
-        Py_XDECREF(cache);
-        return cache;
+        return PyObject_GetAttrString((PyObject *)type, "_weak_cache");
     }
 }
 
@@ -323,26 +318,30 @@ zoneinfo_ZoneInfo_impl(PyTypeObject *type, PyObject *key)
     PyObject *weak_cache = get_weak_cache(state, type);
     instance = PyObject_CallMethod(weak_cache, "get", "O", key, Py_None);
     if (instance == NULL) {
+        Py_DECREF(weak_cache);
         return NULL;
     }
 
     if (instance == Py_None) {
         Py_DECREF(instance);
         PyObject *tmp = zoneinfo_new_instance(state, type, key);
         if (tmp == NULL) {
+            Py_DECREF(weak_cache);
             return NULL;
         }
 
         instance =
             PyObject_CallMethod(weak_cache, "setdefault", "OO", key, tmp);
         Py_DECREF(tmp);
         if (instance == NULL) {
+            Py_DECREF(weak_cache);
             return NULL;
         }
         ((PyZoneInfo_ZoneInfo *)instance)->source = SOURCE_CACHE;
     }
 
     update_strong_cache(state, type, key, instance);
+    Py_DECREF(weak_cache);
     return instance;
 }
 
@@ -504,12 +503,14 @@ zoneinfo_ZoneInfo_clear_cache_impl(PyTypeObject *type, PyTypeObject *cls,
         PyObject *item = NULL;
         PyObject *pop = PyUnicode_FromString("pop");
         if (pop == NULL) {
+            Py_DECREF(weak_cache);
             return NULL;
         }
 
         PyObject *iter = PyObject_GetIter(only_keys);
         if (iter == NULL) {
             Py_DECREF(pop);
+            Py_DECREF(weak_cache);
             return NULL;
         }
 
@@ -534,6 +535,7 @@ zoneinfo_ZoneInfo_clear_cache_impl(PyTypeObject *type, PyTypeObject *cls,
         Py_DECREF(pop);
     }
 
+    Py_DECREF(weak_cache);
     if (PyErr_Occurred()) {
         return NULL;
     }