gh-126138: Fix use-after-free in _asyncio.Task by evil __getattribute__ (#126305)

Co-authored-by: Carol Willing <[email protected]>

Author: Nico-Posada

Misc/NEWS.d/next/Library/2024-11-01-14-31-41.gh-issue-126138.yTniOG.rst

@@ -0,0 +1,3 @@
+Fix a use-after-free crash on :class:`asyncio.Task` objects
+whose underlying coroutine yields an object that implements
+an evil :meth:`~object.__getattribute__`. Patch by Nico Posada.

Modules/_asynciomodule.c

@@ -2967,8 +2967,17 @@ task_step_handle_result_impl(asyncio_state *state, TaskObj *task, PyObject *resu
         if (task->task_must_cancel) {
             PyObject *r;
             int is_true;
+
+            // Beware: An evil `__getattribute__` could
+            // prematurely delete task->task_cancel_msg before the
+            // task is cancelled, thereby causing a UAF crash.
+            //
+            // See https://github.com/python/cpython/issues/126138
+            PyObject *task_cancel_msg = Py_NewRef(task->task_cancel_msg);
             r = PyObject_CallMethodOneArg(result, &_Py_ID(cancel),
-                                             task->task_cancel_msg);
+                                          task_cancel_msg);
+            Py_DECREF(task_cancel_msg);
+
             if (r == NULL) {
                 return NULL;
             }
@@ -3060,8 +3069,17 @@ task_step_handle_result_impl(asyncio_state *state, TaskObj *task, PyObject *resu
         if (task->task_must_cancel) {
             PyObject *r;
             int is_true;
+
+            // Beware: An evil `__getattribute__` could
+            // prematurely delete task->task_cancel_msg before the
+            // task is cancelled, thereby causing a UAF crash.
+            //
+            // See https://github.com/python/cpython/issues/126138
+            PyObject *task_cancel_msg = Py_NewRef(task->task_cancel_msg);
             r = PyObject_CallMethodOneArg(result, &_Py_ID(cancel),
-                                             task->task_cancel_msg);
+                                          task_cancel_msg);
+            Py_DECREF(task_cancel_msg);
+
             if (r == NULL) {
                 return NULL;
             }