gh-126080: fix UAF on task->task_context in task_call_step_soon due to an evil loop.__getattribute__ (#126120)

Author: picnixz

Misc/NEWS.d/next/Library/2024-10-29-10-38-28.gh-issue-126080.qKRBuo.rst

@@ -0,0 +1,3 @@
+Fix a use-after-free crash on :class:`asyncio.Task` objects for which the
+underlying event loop implements an evil :meth:`~object.__getattribute__`.
+Reported by Nico-Posada. Patch by Bénédikt Tran.

Modules/_asynciomodule.c

@@ -2738,7 +2738,11 @@ task_call_step_soon(asyncio_state *state, TaskObj *task, PyObject *arg)
         return -1;
     }
 
-    int ret = call_soon(state, task->task_loop, cb, NULL, task->task_context);
+    // Beware: An evil call_soon could alter task_context.
+    // See: https://github.com/python/cpython/issues/126080.
+    PyObject *task_context = Py_NewRef(task->task_context);
+    int ret = call_soon(state, task->task_loop, cb, NULL, task_context);
+    Py_DECREF(task_context);
     Py_DECREF(cb);
     return ret;
 }