gh-151403: Fix use-after-free when an argv item's __fspath__ mutates args

Lib/test/test_subprocess.py

@@ -3476,6 +3476,45 @@ def test_fork_exec(self):
             if not gc_enabled:
                 gc.disable()
 
+    @support.cpython_only
+    def test_fork_exec_args_concurrent_mutation(self):
+        # An argv item's __fspath__() can run Python that drops the args
+        # sequence's last reference to that item while fork_exec() is still
+        # converting it.  See: https://github.com/python/cpython/issues/151403
+        import _posixsubprocess
+
+        def fork_exec(args):
+            return _posixsubprocess.fork_exec(
+                args, [b"false"],
+                True, (), None, [b"env"],
+                -1, -1, -1, -1,
+                1, 2, 3, 4,
+                True, True, 0,
+                None, None, None, -1,
+                None)
+
+        # __fspath__() returns a non-str/bytes, forcing the error path that
+        # reports the offending item's type.  Clearing the list there drops
+        # the item's last reference: only fork_exec() keeping its own
+        # reference avoids a use-after-free.
+        class Evil:
+            def __fspath__(self):
+                args.clear()
+                return 12345
+        args = [Evil()]
+        with self.assertRaises(TypeError):
+            fork_exec(args)
+
+        # Changing the list length is reported as a RuntimeError before the
+        # next item is converted.
+        class Shrink:
+            def __fspath__(self):
+                args.clear()
+                return "/nonexistent"
+        args = [Shrink(), "/second"]
+        with self.assertRaises(RuntimeError):
+            fork_exec(args)
+
     @support.cpython_only
     def test_fork_exec_sorted_fd_sanity_check(self):
         # Issue #23564: sanity check the fork_exec() fds_to_keep sanity check.

Misc/NEWS.d/next/Library/2026-06-12-22-46-31.gh-issue-151403.DalZWh.rst

@@ -0,0 +1,3 @@
+Fixed a crash in :class:`subprocess.Popen` (and ``_posixsubprocess.fork_exec``)
+when an ``argv`` item's :meth:`~object.__fspath__` concurrently mutates the
+``args`` sequence being converted.

Modules/_posixsubprocess.c

@@ -1090,8 +1090,14 @@ subprocess_fork_exec_impl(PyObject *module, PyObject *process_args,
                 goto cleanup;
             }
             borrowed_arg = PySequence_Fast_GET_ITEM(fast_args, arg_num);
-            if (PyUnicode_FSConverter(borrowed_arg, &converted_arg) == 0)
+            /* borrowed_arg is only borrowed; its __fspath__() may run Python
+               that drops fast_args' last reference to it. */
+            Py_INCREF(borrowed_arg);
+            if (PyUnicode_FSConverter(borrowed_arg, &converted_arg) == 0) {
+                Py_DECREF(borrowed_arg);
                 goto cleanup;
+            }
+            Py_DECREF(borrowed_arg);
             PyTuple_SET_ITEM(converted_args, arg_num, converted_arg);
         }