gh-143637: Fix re-entrant mutation of ancillary data in socket.sendmsg()

Lib/test/test_socket.py

@@ -2222,6 +2222,29 @@ def test_addressinfo_enum(self):
                 source=_socket)
         enum._test_simple_enum(CheckedAddressInfo, socket.AddressInfo)
 
+    @unittest.skipUnless(hasattr(socket.socket, "sendmsg"),"sendmsg not supported")
+    def test_sendmsg_reentrant_ancillary_mutation(self):
+
+        class Mut:
+            def __index__(self):
+                seq.clear()
+                return 0
+
+        seq = [
+            (socket.SOL_SOCKET, Mut(), b'x'),
+            (socket.SOL_SOCKET, 0, b'x'),
+        ]
+
+        left, right = socket.socketpair()
+        self.addCleanup(left.close)
+        self.addCleanup(right.close)
+        self.assertRaises(
+            OSError,
+            left.sendmsg,
+            [b'x'],
+            seq,
+        )
+
 
 @unittest.skipUnless(HAVE_SOCKET_CAN, 'SocketCan required for this test.')
 class BasicCANTest(unittest.TestCase):

Misc/NEWS.d/next/Library/2026-01-17-08-44-25.gh-issue-143637.qyPqDo.rst

@@ -0,0 +1 @@
+Fixed a crash in socket.sendmsg() that could occur if ancillary data is mutated re-entrantly during argument parsing.

Modules/socketmodule.c

@@ -4977,11 +4977,13 @@ _socket_socket_sendmsg_impl(PySocketSockObject *s, PyObject *data_arg,
     if (cmsg_arg == NULL)
         ncmsgs = 0;
     else {
-        if ((cmsg_fast = PySequence_Fast(cmsg_arg,
-                                         "sendmsg() argument 2 must be an "
-                                         "iterable")) == NULL)
+        cmsg_fast = PySequence_Tuple(cmsg_arg);
+        if (cmsg_fast == NULL) {
+            PyErr_SetString(PyExc_TypeError,
+                "sendmsg() argument 2 must be an iterable");
             goto finally;
-        ncmsgs = PySequence_Fast_GET_SIZE(cmsg_fast);
+        }
+        ncmsgs = PyTuple_GET_SIZE(cmsg_fast);
     }
 
 #ifndef CMSG_SPACE
@@ -5001,8 +5003,9 @@ _socket_socket_sendmsg_impl(PySocketSockObject *s, PyObject *data_arg,
     controllen = controllen_last = 0;
     while (ncmsgbufs < ncmsgs) {
         size_t bufsize, space;
+        PyObject *item = PyTuple_GET_ITEM(cmsg_fast, ncmsgbufs);
 
-        if (!PyArg_Parse(PySequence_Fast_GET_ITEM(cmsg_fast, ncmsgbufs),
+        if (!PyArg_Parse(item,
                          "(iiy*):[sendmsg() ancillary data items]",
                          &cmsgs[ncmsgbufs].level,
                          &cmsgs[ncmsgbufs].type,