gh-142637: Fix use-after-free in OrderedDict lookups with a re-entrant __eq__

Lib/test/test_ordered_dict.py

@@ -917,6 +917,40 @@ def __getitem__(self, key):
         msg = "OrderedDict mutated during iteration"
         self.assertRaisesRegex(RuntimeError, msg, od.copy)
 
+    def test_issue142637_clear_in_key_eq(self):
+        # gh-142637: looking up a key must not crash when its __eq__ clears
+        # the dict, which replaces the keys being looked up.
+        class Original:
+            def __hash__(self):
+                return 1
+        class Trigger:
+            def __hash__(self):
+                return 1
+            def __eq__(self, other):
+                od.clear()
+                return True
+
+        def delitem():
+            del od[Trigger()]
+
+        operations = {
+            'del': delitem,
+            'pop': lambda: od.pop(Trigger()),
+            'move_to_end': lambda: od.move_to_end(Trigger()),
+        }
+        for name, operation in operations.items():
+            with self.subTest(operation=name):
+                od = self.OrderedDict()
+                od[Original()] = None
+                # Like for a dict, the key is not found once the dict is cleared.
+                self.assertRaises(KeyError, operation)
+                self.assertEqual(len(od), 0)
+
+        od = self.OrderedDict()
+        od[Original()] = None
+        od[Trigger()] = 'value'
+        self.assertEqual(list(od.values()), ['value'])
+
 
 @unittest.skipUnless(c_coll, 'requires the C version of the collections module')
 class CPythonOrderedDictTests(OrderedDictTests,

Misc/NEWS.d/next/Library/2026-10-22-23-45-00.gh-issue-142637.k3Vq9z.rst

@@ -0,0 +1,3 @@
+Fix a use-after-free in :class:`collections.OrderedDict` when a key's
+:meth:`~object.__eq__` method clears the dictionary during a lookup, for
+example in ``del od[key]``, :meth:`!pop` and :meth:`!move_to_end`.

Objects/odictobject.c

@@ -538,7 +538,6 @@ _odict_get_index_raw(PyODictObject *od, PyObject *key, Py_hash_t hash)
 {
     _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(od);
     PyObject *value = NULL;
-    PyDictKeysObject *keys = ((PyDictObject *)od)->ma_keys;
     Py_ssize_t ix;
 #ifdef Py_GIL_DISABLED
     ix = _Py_dict_lookup_threadsafe((PyDictObject *)od, key, hash, &value);
@@ -547,7 +546,9 @@ _odict_get_index_raw(PyODictObject *od, PyObject *key, Py_hash_t hash)
     ix = _Py_dict_lookup((PyDictObject *)od, key, hash, &value);
 #endif
     if (ix == DKIX_EMPTY) {
-        return keys->dk_nentries;  /* index of new entry */
+        /* A re-entrant __eq__ may have replaced the keys during the lookup,
+           so they must be read again here. */
+        return ((PyDictObject *)od)->ma_keys->dk_nentries;  /* index of new entry */
     }
     if (ix < 0)
         return -1;
@@ -602,17 +603,25 @@ _odict_get_index(PyODictObject *od, PyObject *key, Py_hash_t hash)
     PyDictKeysObject *keys;
 
     assert(key != NULL);
-    keys = ((PyDictObject *)od)->ma_keys;
+    for (;;) {
+        keys = ((PyDictObject *)od)->ma_keys;
+
+        /* Ensure od_fast_nodes and dk_entries are in sync. */
+        if (od->od_resize_sentinel != keys ||
+            od->od_fast_nodes_size != (ONE << (keys->dk_log2_size))) {
+            int resize_res = _odict_resize(od);
+            if (resize_res < 0)
+                return -1;
+        }
 
-    /* Ensure od_fast_nodes and dk_entries are in sync. */
-    if (od->od_resize_sentinel != keys ||
-        od->od_fast_nodes_size != (ONE << (keys->dk_log2_size))) {
-        int resize_res = _odict_resize(od);
-        if (resize_res < 0)
-            return -1;
+        Py_ssize_t index = _odict_get_index_raw(od, key, hash);
+        if (index < 0 || keys == ((PyDictObject *)od)->ma_keys) {
+            return index;
+        }
+        /* The keys were replaced while looking up the key, e.g. by a
+           re-entrant __eq__ clearing the dict: od_fast_nodes no longer
+           matches them, so synchronize it and look the key up again. */
     }
-
-    return _odict_get_index_raw(od, key, hash);
 }
 
 /* Returns NULL if there was some error or the key was not found. */