gh-151370: Fix marshal.dumps() crash on concurrent container mutation

Lib/test/test_marshal.py

@@ -243,6 +243,51 @@ def test_array(self):
         new = marshal.loads(marshal.dumps(a))
         self.assertEqual(new, b"abc")
 
+    def test_concurrent_buffer_mutation(self):
+        # An item's __buffer__() runs Python while the container is being
+        # marshalled, so the container can be concurrently mutated (simulated
+        # here by mutating it inside __buffer__).
+        # See: https://github.com/python/cpython/issues/151370
+        def make(kind):
+            # The item is only referenced from the container, so clearing the
+            # container inside __buffer__ drops its last reference to it.
+            class Item:
+                def __buffer__(self, flags):
+                    container.clear()
+                    return memoryview(bytearray(4))
+            item = Item()
+            if kind is dict:
+                container = {item: 1, 2: item}
+            elif kind is set:
+                container = {item, b'other'}
+            else:
+                container = [item, 1, 2]
+            return container
+
+        # A size change while marshalling is reported as a RuntimeError.
+        for kind in (list, dict, set):
+            with self.subTest(kind=kind):
+                self.assertRaises(RuntimeError, marshal.dumps, make(kind))
+
+        # The length is unchanged, so the size recheck cannot fire: only
+        # keeping the item alive across its __buffer__() avoids the crash.
+        class Replacer:
+            def __buffer__(self, flags):
+                container[0] = b'.'
+                return memoryview(bytearray(4))
+        container = [Replacer()]
+        result = marshal.loads(marshal.dumps(container))
+        self.assertEqual(result, [b'\x00' * 4])
+
+        # Growing the set during marshalling is reported too, rather than
+        # writing past the buffer pre-sized to the original length.
+        class Grower:
+            def __buffer__(self, flags):
+                grown.add(b'grown')
+                return memoryview(bytearray(4))
+        grown = {Grower(), 1, 2}
+        self.assertRaises(RuntimeError, marshal.dumps, grown)
+
 
 class BugsTestCase(unittest.TestCase):
     def test_bug_5888452(self):

Misc/NEWS.d/next/Library/2026-06-12-01-10-31.gh-issue-151370.NIfHZ6.rst

@@ -0,0 +1,4 @@
+Fixed a crash (use-after-free) in :func:`marshal.dumps` that could occur if an
+item's :meth:`~object.__buffer__` concurrently mutates the list, dict, or set
+being serialized.  A resulting change in the container's size is now reported
+as a :exc:`RuntimeError`.

Python/marshal.c

@@ -106,6 +106,7 @@ module marshal
 #define WFERR_NESTEDTOODEEP 2
 #define WFERR_NOMEMORY 3
 #define WFERR_CODE_NOT_ALLOWED 4
+#define WFERR_CHANGED_SIZE 5
 
 typedef struct {
     FILE *fp;
@@ -489,8 +490,13 @@ w_object(PyObject *v, WFILE *p)
     else if (v == Py_True) {
         w_byte(TYPE_TRUE, p);
     }
-    else if (!w_ref(v, &flag, p))
+    else if (!w_ref(v, &flag, p)) {
+        /* Keep v alive while it is serialized: this may run Python (an item's
+           __buffer__) that drops the caller's last reference to v. */
+        Py_INCREF(v);
         w_complex_object(v, flag, p);
+        Py_DECREF(v);
+    }
 
     p->depth--;
 }
@@ -603,6 +609,12 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
         W_SIZE(n, p);
         for (i = 0; i < n; i++) {
             w_object(PyList_GET_ITEM(v, i), p);
+            /* w_object() may run Python (e.g. an item's __buffer__) that
+               resizes the list, leaving the captured size and index stale. */
+            if (PyList_GET_SIZE(v) != n) {
+                p->error = WFERR_CHANGED_SIZE;
+                return;
+            }
         }
     }
     else if (PyAnyDict_CheckExact(v)) {
@@ -621,10 +633,23 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
             W_TYPE(TYPE_DICT, p);
         }
         /* This one is NULL object terminated! */
+        n = PyDict_GET_SIZE(v);
         pos = 0;
         while (PyDict_Next(v, &pos, &key, &value)) {
+            /* key and value are only borrowed; an item's __buffer__() may run
+               Python that drops the dict's last reference to them. */
+            Py_INCREF(key);
+            Py_INCREF(value);
             w_object(key, p);
             w_object(value, p);
+            Py_DECREF(key);
+            Py_DECREF(value);
+            /* That Python may also have resized the dict, making the
+               PyDict_Next() position stale. */
+            if (PyDict_GET_SIZE(v) != n) {
+                p->error = WFERR_CHANGED_SIZE;
+                return;
+            }
         }
         w_object((PyObject *)NULL, p);
         if (PyFrozenDict_CheckExact(v)) {
@@ -654,6 +679,13 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
         Py_ssize_t i = 0;
         Py_BEGIN_CRITICAL_SECTION(v);
         while (_PySet_NextEntryRef(v, &pos, &value, &hash)) {
+            /* A previously serialized element's __buffer__ may have grown the
+               set; i >= n stops a write past the pre-sized pairs list. */
+            if (i >= n) {
+                p->error = WFERR_CHANGED_SIZE;
+                Py_DECREF(value);
+                break;
+            }
             PyObject *dump = _PyMarshal_WriteObjectToString(value,
                                     p->version, p->allow_code);
             if (dump == NULL) {
@@ -669,11 +701,18 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
             PyList_SET_ITEM(pairs, i++, pair);
         }
         Py_END_CRITICAL_SECTION();
-        if (p->error == WFERR_UNMARSHALLABLE || p->error == WFERR_NOMEMORY) {
+        if (p->error == WFERR_UNMARSHALLABLE || p->error == WFERR_NOMEMORY
+                || p->error == WFERR_CHANGED_SIZE) {
+            Py_DECREF(pairs);
+            return;
+        }
+        if (i != n || PySet_GET_SIZE(v) != n) {
+            /* The set was mutated while being marshalled (e.g. cleared by the
+               last element's __buffer__, or an under-yield). */
+            p->error = WFERR_CHANGED_SIZE;
             Py_DECREF(pairs);
             return;
         }
-        assert(i == n);
         if (PyList_Sort(pairs)) {
             p->error = WFERR_NOMEMORY;
             Py_DECREF(pairs);
@@ -1941,6 +1980,10 @@ _PyMarshal_WriteObjectToString(PyObject *x, int version, int allow_code)
             PyErr_SetString(PyExc_ValueError,
                             "marshalling code objects is disallowed");
             break;
+        case WFERR_CHANGED_SIZE:
+            PyErr_SetString(PyExc_RuntimeError,
+                            "dict, list or set changed size during iteration");
+            break;
         default:
         case WFERR_UNMARSHALLABLE:
             PyErr_SetString(PyExc_ValueError,