gh-143004: Fix possible use-after-free in collections.Counter.update() (GH-143044)

This happened when the Counter was mutated when incrementing
the value for an existing key.

Author: Kaushalt2004

Lib/test/test_collections.py

@@ -2135,6 +2135,19 @@ def test_basics(self):
         self.assertEqual(c.setdefault('e', 5), 5)
         self.assertEqual(c['e'], 5)
 
+    def test_update_reentrant_add_clears_counter(self):
+        c = Counter()
+        key = object()
+
+        class Evil(int):
+            def __add__(self, other):
+                c.clear()
+                return NotImplemented
+
+        c[key] = Evil()
+        c.update([key])
+        self.assertEqual(c[key], 1)
+
     def test_init(self):
         self.assertEqual(list(Counter(self=42).items()), [('self', 42)])
         self.assertEqual(list(Counter(iterable=42).items()), [('iterable', 42)])

Misc/NEWS.d/next/Library/2025-12-22-00-00-00.gh-issue-143004.uaf-counter.rst

@@ -0,0 +1,2 @@
+Fix a potential use-after-free in :meth:`collections.Counter.update` when user code
+mutates the Counter during an update.

Modules/_collectionsmodule.c

@@ -2577,7 +2577,12 @@ _collections__count_elements_impl(PyObject *module, PyObject *mapping,
                 if (_PyDict_SetItem_KnownHash(mapping, key, one, hash) < 0)
                     goto done;
             } else {
+                /* oldval is a borrowed reference.  Keep it alive across
+                   PyNumber_Add(), which can execute arbitrary user code and
+                   mutate (or even clear) the underlying dict. */
+                Py_INCREF(oldval);
                 newval = PyNumber_Add(oldval, one);
+                Py_DECREF(oldval);
                 if (newval == NULL)
                     goto done;
                 if (_PyDict_SetItem_KnownHash(mapping, key, newval, hash) < 0)