bpo-38610: Fix possible crashes in several list methods (pythonGH-17022)

Hold strong references to list elements while calling PyObject_RichCompareBool().

Author: ZackerySpytz

Lib/test/test_list.py

@@ -151,5 +151,31 @@ class L(list): pass
         with self.assertRaises(TypeError):
             (3,) + L([1,2])
 
+    def test_count_index_remove_crashes(self):
+        # bpo-38610: The count(), index(), and remove() methods were not
+        # holding strong references to list elements while calling
+        # PyObject_RichCompareBool().
+        class X:
+            def __eq__(self, other):
+                lst.clear()
+                return NotImplemented
+
+        lst = [X()]
+        with self.assertRaises(ValueError):
+            lst.index(lst)
+
+        class L(list):
+            def __eq__(self, other):
+                str(other)
+                return NotImplemented
+
+        lst = L([X()])
+        lst.count(lst)
+
+        lst = L([X()])
+        with self.assertRaises(ValueError):
+            lst.remove(lst)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2019-10-31-14-30-39.bpo-38610.fHdVMS.rst

@@ -0,0 +1,2 @@
+Fix possible crashes in several list methods by holding strong references to
+list elements when calling :c:func:`PyObject_RichCompareBool`.

Objects/listobject.c

@@ -2164,7 +2164,10 @@ listindex(PyListObject *self, PyObject *args)
             stop = 0;
     }
     for (i = start; i < stop && i < Py_SIZE(self); i++) {
-        int cmp = PyObject_RichCompareBool(self->ob_item[i], v, Py_EQ);
+        PyObject *obj = self->ob_item[i];
+        Py_INCREF(obj);
+        int cmp = PyObject_RichCompareBool(obj, v, Py_EQ);
+        Py_DECREF(obj);
         if (cmp > 0)
             return PyLong_FromSsize_t(i);
         else if (cmp < 0)
@@ -2181,7 +2184,10 @@ listcount(PyListObject *self, PyObject *v)
     Py_ssize_t i;
 
     for (i = 0; i < Py_SIZE(self); i++) {
-        int cmp = PyObject_RichCompareBool(self->ob_item[i], v, Py_EQ);
+        PyObject *obj = self->ob_item[i];
+        Py_INCREF(obj);
+        int cmp = PyObject_RichCompareBool(obj, v, Py_EQ);
+        Py_DECREF(obj);
         if (cmp > 0)
             count++;
         else if (cmp < 0)
@@ -2196,7 +2202,10 @@ listremove(PyListObject *self, PyObject *v)
     Py_ssize_t i;
 
     for (i = 0; i < Py_SIZE(self); i++) {
-        int cmp = PyObject_RichCompareBool(self->ob_item[i], v, Py_EQ);
+        PyObject *obj = self->ob_item[i];
+        Py_INCREF(obj);
+        int cmp = PyObject_RichCompareBool(obj, v, Py_EQ);
+        Py_DECREF(obj);
         if (cmp > 0) {
             if (list_ass_slice(self, i, i+1,
                                (PyObject *)NULL) == 0)