[Python-checkins] bpo-39382: Avoid dangling object use in abstract_issubclass() (GH-18530)

[Yonatan Goldschmidt]

https://github.com/python/cpython/commit/1c56f8ffad44478b4214a2bf8eb7cf51c28a347a
commit: 1c56f8ffad44478b4214a2bf8eb7cf51c28a347a
branch: master
author: Yonatan Goldschmidt <yon.goldschmidt at gmail.com>
committer: GitHub <noreply at github.com>
date: 2020-02-22T15:11:48+02:00
summary:

bpo-39382: Avoid dangling object use in abstract_issubclass() (GH-18530)

Hold reference of __bases__ tuple until tuple item is done with, because by
dropping the reference the item may be destroyed.

files:
A Misc/NEWS.d/next/Core and Builtins/2020-02-18-01-40-13.bpo-39382.OLSJu9.rst
M Lib/test/test_isinstance.py
M Misc/ACKS
M Objects/abstract.c

diff --git a/Lib/test/test_isinstance.py b/Lib/test/test_isinstance.py
index 65751ab916855..53639e984e48a 100644
--- a/Lib/test/test_isinstance.py
+++ b/Lib/test/test_isinstance.py
@@ -251,6 +251,27 @@ def test_isinstance_recursion_limit(self):
         # blown
         self.assertRaises(RecursionError, blowstack, isinstance, '', str)
 
+    def test_issubclass_refcount_handling(self):
+        # bpo-39382: abstract_issubclass() didn't hold item reference while
+        # peeking in the bases tuple, in the single inheritance case.
+        class A:
+            @property
+            def __bases__(self):
+                return (int, )
+
+        class B:
+            def __init__(self):
+                # setting this here increases the chances of exhibiting the bug,
+                # probably due to memory layout changes.
+                self.x = 1
+
+            @property
+            def __bases__(self):
+                return (A(), )
+
+        self.assertEqual(True, issubclass(B(), int))
+
+
 def blowstack(fxn, arg, compare_to):
     # Make sure that calling isinstance with a deeply nested tuple for its
     # argument will raise RecursionError eventually.
diff --git a/Misc/ACKS b/Misc/ACKS
index f329a2d4a7d33..fe24a5636ccc2 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -594,6 +594,7 @@ Karan Goel
 Jeroen Van Goey
 Christoph Gohlke
 Tim Golden
+Yonatan Goldschmidt
 Mark Gollahon
 Guilherme Gonçalves
 Tiago Gonçalves
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-02-18-01-40-13.bpo-39382.OLSJu9.rst b/Misc/NEWS.d/next/Core and Builtins/2020-02-18-01-40-13.bpo-39382.OLSJu9.rst
new file mode 100644
index 0000000000000..605f4c8e5dfd1
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-02-18-01-40-13.bpo-39382.OLSJu9.rst	
@@ -0,0 +1,3 @@
+Fix a use-after-free in the single inheritance path of ``issubclass()``, when
+the ``__bases__`` of an object has a single reference, and so does its first item.
+Patch by Yonatan Goldschmidt.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index f0e01f7691bb3..de5652f3e685f 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -2379,9 +2379,16 @@ abstract_issubclass(PyObject *derived, PyObject *cls)
     int r = 0;
 
     while (1) {
-        if (derived == cls)
+        if (derived == cls) {
+            Py_XDECREF(bases); /* See below comment */
             return 1;
-        bases = abstract_get_bases(derived);
+        }
+        /* Use XSETREF to drop bases reference *after* finishing with
+           derived; bases might be the only reference to it.
+           XSETREF is used instead of SETREF, because bases is NULL on the
+           first iteration of the loop.
+        */
+        Py_XSETREF(bases, abstract_get_bases(derived));
         if (bases == NULL) {
             if (PyErr_Occurred())
                 return -1;
@@ -2395,7 +2402,6 @@ abstract_issubclass(PyObject *derived, PyObject *cls)
         /* Avoid recursivity in the single inheritance case */
         if (n == 1) {
             derived = PyTuple_GET_ITEM(bases, 0);
-            Py_DECREF(bases);
             continue;
         }
         for (i = 0; i < n; i++) {