gh-152405: Avoid exposing mappingproxy backing mapping

Author: zainnadeem786

Lib/test/test_types.py

@@ -1180,6 +1180,28 @@ def test_instantiation(self):
 class MappingProxyTests(unittest.TestCase):
     mappingproxy = types.MappingProxyType
 
+    class MappingWithoutCopy(collections.abc.Mapping):
+        def __init__(self, mapping):
+            self.mapping = mapping
+
+        def __getitem__(self, key):
+            return self.mapping[key]
+
+        def __iter__(self):
+            return iter(self.mapping)
+
+        def __len__(self):
+            return len(self.mapping)
+
+    class RecordingOperand:
+        def __eq__(self, other):
+            self.other = other
+            return None
+
+        def __ror__(self, other):
+            self.other = other
+            return None
+
     def test_constructor(self):
         class userdict(dict):
             pass
@@ -1381,6 +1403,70 @@ def test_union(self):
         self.assertDictEqual(mapping, {'a': 0, 'b': 1, 'c': 2})
         self.assertDictEqual(other, {'c': 3, 'p': 0})
 
+    def test_richcompare_does_not_expose_mapping(self):
+        mapping = {}
+        view = self.mappingproxy(mapping)
+
+        class Sneaky:
+            def __eq__(self, other):
+                other['x'] = 42
+                return None
+
+        self.assertIsNone(view == Sneaky())
+        self.assertEqual(mapping, {})
+
+        other = self.RecordingOperand()
+        view = self.mappingproxy(self.MappingWithoutCopy(mapping))
+        self.assertIsNone(view == other)
+        self.assertIs(type(other.other), self.mappingproxy)
+
+    def test_union_does_not_expose_mapping(self):
+        mapping = {}
+        view = self.mappingproxy(mapping)
+        mappingproxy = self.mappingproxy
+
+        class SneakyRor:
+            def __ror__(self, other):
+                other['x'] = 42
+                return None
+
+        class SneakyOr:
+            def __or__(self, other):
+                if type(other) is mappingproxy:
+                    return NotImplemented
+                other['y'] = 42
+                return None
+
+        self.assertIsNone(view | SneakyRor())
+        self.assertEqual(mapping, {})
+        self.assertIsNone(SneakyOr() | view)
+        self.assertEqual(mapping, {})
+
+        other = self.RecordingOperand()
+        view = self.mappingproxy(self.MappingWithoutCopy(mapping))
+        self.assertIsNone(view | other)
+        self.assertIs(type(other.other), self.mappingproxy)
+
+    def test_operator_dispatch_does_not_expose_type_dict(self):
+        code = textwrap.dedent("""
+            class SneakyEq:
+                def __eq__(self, other):
+                    other['__mappingproxy_test_eq__'] = lambda self: None
+                    return None
+
+            class SneakyRor:
+                def __ror__(self, other):
+                    other['__mappingproxy_test_or__'] = lambda self: None
+                    return None
+
+            vars(list) == SneakyEq()
+            vars(dict) | SneakyRor()
+
+            assert not hasattr(list, '__mappingproxy_test_eq__')
+            assert not hasattr(dict, '__mappingproxy_test_or__')
+        """)
+        assert_python_ok("-c", code)
+
     def test_hash(self):
         class HashableDict(dict):
             def __hash__(self):

Misc/NEWS.d/next/Core_and_Builtins/2026-06-27-21-30-00.gh-issue-152405.XYp8rQ.rst

@@ -0,0 +1,4 @@
+Prevent ``mappingproxy`` operator dispatch from exposing the underlying mapping
+during rich comparison and union operations. This fixes a case where reflected
+operator methods could access and mutate internal dictionaries, including those
+of built-in types.

Objects/descrobject.c

@@ -1061,16 +1061,50 @@ static PyMappingMethods mappingproxy_as_mapping = {
     0,                                          /* mp_ass_subscript */
 };
 
+/* Use a copy of proxied mappings for operator dispatch, so reflected
+   methods cannot access the underlying mapping. */
+static PyObject *
+mappingproxy_copy_mapping(PyObject *mapping)
+{
+    PyObject *copy_method;
+    int res = PyObject_GetOptionalAttr(mapping, &_Py_ID(copy), &copy_method);
+    if (res < 0) {
+        return NULL;
+    }
+    if (res == 0) {
+        Py_RETURN_NOTIMPLEMENTED;
+    }
+
+    PyObject *copy = PyObject_CallNoArgs(copy_method);
+    Py_DECREF(copy_method);
+    return copy;
+}
+
+static PyObject *
+mappingproxy_as_operand(PyObject *operand)
+{
+    if (PyObject_TypeCheck(operand, &PyDictProxy_Type)) {
+        return mappingproxy_copy_mapping(((mappingproxyobject *)operand)->mapping);
+    }
+    return Py_NewRef(operand);
+}
+
 static PyObject *
 mappingproxy_or(PyObject *left, PyObject *right)
 {
-    if (PyObject_TypeCheck(left, &PyDictProxy_Type)) {
-        left = ((mappingproxyobject*)left)->mapping;
+    left = mappingproxy_as_operand(left);
+    if (left == NULL || left == Py_NotImplemented) {
+        return left;
     }
-    if (PyObject_TypeCheck(right, &PyDictProxy_Type)) {
-        right = ((mappingproxyobject*)right)->mapping;
+    right = mappingproxy_as_operand(right);
+    if (right == NULL || right == Py_NotImplemented) {
+        Py_DECREF(left);
+        return right;
     }
-    return PyNumber_Or(left, right);
+    PyObject *result = PyNumber_Or(left, right);
+    Py_DECREF(left);
+    Py_DECREF(right);
+    return result;
 }
 
 static PyObject *
@@ -1234,7 +1268,13 @@ mappingproxy_richcompare(PyObject *self, PyObject *w, int op)
 {
     mappingproxyobject *v = (mappingproxyobject *)self;
     if (op == Py_EQ || op == Py_NE) {
-        return PyObject_RichCompare(v->mapping, w, op);
+        PyObject *mapping = mappingproxy_copy_mapping(v->mapping);
+        if (mapping == NULL || mapping == Py_NotImplemented) {
+            return mapping;
+        }
+        PyObject *result = PyObject_RichCompare(mapping, w, op);
+        Py_DECREF(mapping);
+        return result;
     }
     Py_RETURN_NOTIMPLEMENTED;
 }