bpo-43838: Use a copy to delegate types.MappingProxyType operators

Doc/library/types.rst

@@ -369,6 +369,12 @@ Standard names are defined for the following types:
       Updated to support the new union (``|``) operator from :pep:`584`, which
       simply delegates to the underlying mapping.
 
+   .. versionchanged:: 3.10
+
+      To avoid exposing the actual proxied object to arbitrary code, union and
+      rich comparison operations now delegate to a copy of the underlying
+      mapping instead.
+
    .. describe:: key in proxy
 
       Return ``True`` if the underlying mapping has a key *key*, else

Lib/test/test_types.py

@@ -1131,6 +1131,47 @@ def test_union(self):
         self.assertDictEqual(mapping, {'a': 0, 'b': 1, 'c': 2})
         self.assertDictEqual(other, {'c': 3, 'p': 0})
 
+    def test_delegated_operators(self):
+        class MyDict(dict):
+            def __eq__(self, other):
+                return super().__eq__(other)
+            def __or__(self, other):
+                return super().__or__(other)
+            def __ror__(self, other):
+                return super().__ror__(other)
+        dict_a = {0: "a"}
+        dict_b = {0: "b"}
+        mydict_a = MyDict(dict_a)
+        mydict_b = MyDict(dict_b)
+        view_dict_a = self.mappingproxy(dict_a)
+        view_mydict_a = self.mappingproxy(mydict_a)
+        view_view_dict_a = self.mappingproxy(view_dict_a)
+        a_mappings = (dict_a, mydict_a)
+        b_mappings = (dict_b, mydict_b)
+        a_views = (view_dict_a, view_mydict_a, view_view_dict_a)
+        for view in a_views:
+            for mapping in a_mappings + a_views:
+                self.assertDictEqual(view | mapping, dict_a)
+                self.assertDictEqual(mapping | view, dict_a)
+                self.assertEqual(view, mapping)
+                self.assertEqual(mapping, view)
+            for mapping in b_mappings:
+                self.assertDictEqual(view | mapping, dict_b)
+                self.assertDictEqual(mapping | view, dict_a)
+                self.assertNotEqual(view, mapping)
+                self.assertNotEqual(mapping, view)
+
+    def test_bpo_43838(self):
+        mapping = {}
+        proxy = self.mappingproxy(mapping)
+        class Sneaky:
+            def __eq__(self, other):
+                other['x'] = 42
+                return None
+        self.assertIs(proxy == Sneaky(), None)
+        self.assertDictEqual(mapping, {})
+        self.assertEqual(proxy, {})
+
 
 class ClassCreationTests(unittest.TestCase):
 

Misc/NEWS.d/next/Core and Builtins/2021-07-22-16-10-14.bpo-43838.a5j8Q3.rst

@@ -0,0 +1,3 @@
+To avoid exposing the actual proxied object to arbitrary code, union and
+rich comparison operations on :class:`types.MappingProxyType` now delegate
+to a copy of the underlying mapping instead.

Objects/descrobject.c

@@ -1032,13 +1032,36 @@ static PyMappingMethods mappingproxy_as_mapping = {
 static PyObject *
 mappingproxy_or(PyObject *left, PyObject *right)
 {
+    // bpo-43838: We can't just return PyNumber_Or(left->mapping, right) or
+    // PyNumber_Or(left, right->mapping) here, because the operator dispatch dance can
+    // expose our hidden mapping to arbitrary code. Perform the operation with a copy of
+    // the mapping instead:
+    _Py_IDENTIFIER(copy);
     if (PyObject_TypeCheck(left, &PyDictProxy_Type)) {
         left = ((mappingproxyobject*)left)->mapping;
+        left = _PyObject_CallMethodIdNoArgs(left, &PyId_copy);
+        if (left == NULL) {
+            return NULL;
+        }
+    }
+    else {
+        Py_INCREF(left);
     }
     if (PyObject_TypeCheck(right, &PyDictProxy_Type)) {
         right = ((mappingproxyobject*)right)->mapping;
+        right = _PyObject_CallMethodIdNoArgs(right, &PyId_copy);
+        if (right == NULL) {
+            Py_DECREF(left);
+            return NULL;
+        }
     }
-    return PyNumber_Or(left, right);
+    else {
+        Py_INCREF(right);
+    }
+    PyObject *result = PyNumber_Or(left, right);
+    Py_DECREF(left);
+    Py_DECREF(right);
+    return result;
 }
 
 static PyObject *
@@ -1188,7 +1211,17 @@ mappingproxy_traverse(PyObject *self, visitproc visit, void *arg)
 static PyObject *
 mappingproxy_richcompare(mappingproxyobject *v, PyObject *w, int op)
 {
-    return PyObject_RichCompare(v->mapping, w, op);
+    // bpo-43838: We can't just return PyObject_RichCompare(v->mapping, w, op) here,
+    // because the operator dispatch dance can expose our hidden mapping to arbitrary
+    // code. Perform the operation with a copy of the mapping instead:
+    _Py_IDENTIFIER(copy);
+    PyObject *copy = _PyObject_CallMethodIdNoArgs(v->mapping, &PyId_copy);
+    if (copy == NULL) {
+        return NULL;
+    }
+    PyObject *result = PyObject_RichCompare(copy, w, op);
+    Py_DECREF(copy);
+    return result;
 }
 
 static int