bpo-30828: Fix out of bounds write in `asyncio.CFuture.remove_done_callback() (#2569)

Lib/test/test_asyncio/test_futures.py

@@ -593,7 +593,7 @@ class BaseFutureDoneCallbackTests():
 
         fut.remove_done_callback(evil())
 
-    def test_schedule_callbacks_list_mutation(self):
+    def test_schedule_callbacks_list_mutation_1(self):
         # see http://bugs.python.org/issue28963 for details
 
         def mut(f):
@@ -606,6 +606,28 @@ class BaseFutureDoneCallbackTests():
         fut.set_result(1)
         test_utils.run_briefly(self.loop)
 
+    def test_schedule_callbacks_list_mutation_2(self):
+        # see http://bugs.python.org/issue30828 for details
+
+        fut = self._new_future()
+        fut.add_done_callback(str)
+
+        for _ in range(63):
+            fut.add_done_callback(id)
+
+        max_extra_cbs = 100
+        extra_cbs = 0
+
+        class evil:
+            def __eq__(self, other):
+                nonlocal extra_cbs
+                extra_cbs += 1
+                if extra_cbs < max_extra_cbs:
+                    fut.add_done_callback(id)
+                return False
+
+        fut.remove_done_callback(evil())
+
 
 @unittest.skipUnless(hasattr(futures, '_CFuture'),
                      'requires the C _asyncio module')

Misc/NEWS.d/next/Library/2017-07-04-13-10-52.bpo-30828.CLvEvV.rst

@@ -0,0 +1 @@
+Fix out of bounds write in `asyncio.CFuture.remove_done_callback()`.

Modules/_asynciomodule.c

@@ -532,9 +532,16 @@ _asyncio_Future_remove_done_callback(FutureObj *self, PyObject *fn)
             goto fail;
         }
         if (ret == 0) {
-            Py_INCREF(item);
-            PyList_SET_ITEM(newlist, j, item);
-            j++;
+            if (j < len) {
+                Py_INCREF(item);
+                PyList_SET_ITEM(newlist, j, item);
+                j++;
+            }
+            else {
+                if (PyList_Append(newlist, item)) {
+                    goto fail;
+                }
+            }
         }
     }