[3.9] bpo-30570: Use Py_EnterRecursiveCall() in issubclass() (GH-29048)

Lib/test/test_isinstance.py

@@ -281,6 +281,36 @@ def __bases__(self):
         self.assertRaises(RecursionError, issubclass, int, X())
         self.assertRaises(RecursionError, isinstance, 1, X())
 
+    def test_infinite_recursion_via_bases_tuple(self):
+        """Regression test for bpo-30570."""
+        class Failure(object):
+            def __getattr__(self, attr):
+                return (self, None)
+
+        with self.assertRaises(RecursionError):
+            issubclass(Failure(), int)
+
+    def test_infinite_cycle_in_bases(self):
+        """Regression test for bpo-30570."""
+        class X:
+            @property
+            def __bases__(self):
+                return (self, self, self)
+        self.assertRaises(RecursionError, issubclass, X(), int)
+
+    def test_infinitely_many_bases(self):
+        """Regression test for bpo-30570."""
+        class X:
+            def __getattr__(self, attr):
+                self.assertEqual(attr, "__bases__")
+                class A:
+                    pass
+                class B:
+                    pass
+                A.__getattr__ = B.__getattr__ = X.__getattr__
+                return (A(), B())
+        self.assertRaises(RecursionError, issubclass, X(), int)
+
 
 def blowstack(fxn, arg, compare_to):
     # Make sure that calling isinstance with a deeply nested tuple for its

Misc/NEWS.d/next/Core and Builtins/2021-10-19-01-04-08.bpo-30570._G30Ms.rst

@@ -0,0 +1 @@
+Fixed a crash in ``issubclass()`` from infinite recursion when searching pathological ``__bases__`` tuples.

Objects/abstract.c

@@ -2417,14 +2417,22 @@ abstract_issubclass(PyObject *derived, PyObject *cls)
             derived = PyTuple_GET_ITEM(bases, 0);
             continue;
         }
-        for (i = 0; i < n; i++) {
-            r = abstract_issubclass(PyTuple_GET_ITEM(bases, i), cls);
-            if (r != 0)
-                break;
-        }
+        break;
+    }
+    assert(n >= 2);
+    if (Py_EnterRecursiveCall(" in __issubclass__")) {
         Py_DECREF(bases);
-        return r;
+        return -1;
     }
+    for (i = 0; i < n; i++) {
+        r = abstract_issubclass(PyTuple_GET_ITEM(bases, i), cls);
+        if (r != 0) {
+            break;
+        }
+    }
+    Py_LeaveRecursiveCall();
+    Py_DECREF(bases);
+    return r;
 }
 
 static int