Closes issue 11133. Fixes two cases where inspect.getattr_static could trigger code execution

Doc/library/inspect.rst

@@ -589,13 +589,10 @@ but avoids executing code when it fetches attributes.
    that raise AttributeError). It can also return descriptors objects
    instead of instance members.
 
-   .. versionadded:: 3.2
+   If the instance `__dict__` is shadowed by another member (for example a
+   property) then this function will be unable to find instance members.
 
-The only known case that can cause `getattr_static` to trigger code execution,
-and cause it to return incorrect results (or even break), is where a class uses
-:data:`~object.__slots__` and provides a `__dict__` member using a property or
-descriptor. If you find other cases please report them so they can be fixed
-or documented.
+   .. versionadded:: 3.2
 
 `getattr_static` does not resolve descriptors, for example slot descriptors or
 getset descriptors on objects implemented in C. The descriptor object

Lib/inspect.py

@@ -1069,15 +1069,16 @@ def _check_instance(obj, attr):
         instance_dict = object.__getattribute__(obj, "__dict__")
     except AttributeError:
         pass
-    return instance_dict.get(attr, _sentinel)
+    return dict.get(instance_dict, attr, _sentinel)
 
 
 def _check_class(klass, attr):
     for entry in _static_getmro(klass):
-        try:
-            return entry.__dict__[attr]
-        except KeyError:
-            pass
+        if not _shadowed_dict(type(entry)):
+            try:
+                return entry.__dict__[attr]
+            except KeyError:
+                pass
     return _sentinel
 
 def _is_type(obj):
@@ -1087,6 +1088,19 @@ def _is_type(obj):
         return False
     return True
 
+def _shadowed_dict(klass):
+    dict_attr = type.__dict__["__dict__"]
+    for entry in _static_getmro(klass):
+        try:
+            class_dict = dict_attr.__get__(entry)["__dict__"]
+        except KeyError:
+            pass
+        else:
+            if not (type(class_dict) is types.GetSetDescriptorType and
+                    class_dict.__name__ == "__dict__" and
+                    class_dict.__objclass__ is entry):
+                return True
+    return False
 
 def getattr_static(obj, attr, default=_sentinel):
     """Retrieve attributes without triggering dynamic lookup via the
@@ -1101,8 +1115,9 @@ def getattr_static(obj, attr, default=_sentinel):
     """
     instance_result = _sentinel
     if not _is_type(obj):
-        instance_result = _check_instance(obj, attr)
         klass = type(obj)
+        if not _shadowed_dict(klass):
+            instance_result = _check_instance(obj, attr)
     else:
         klass = obj
 

Lib/test/test_inspect.py

@@ -906,6 +906,53 @@ class TestGetattrStatic(unittest.TestCase):
         self.assertEqual(inspect.getattr_static(Something(), 'foo'), 3)
         self.assertEqual(inspect.getattr_static(Something, 'foo'), 3)
 
+    def test_dict_as_property(self):
+        test = self
+        test.called = False
+
+        class Foo(dict):
+            a = 3
+            @property
+            def __dict__(self):
+                test.called = True
+                return {}
+
+        foo = Foo()
+        foo.a = 4
+        self.assertEqual(inspect.getattr_static(foo, 'a'), 3)
+        self.assertFalse(test.called)
+
+    def test_custom_object_dict(self):
+        test = self
+        test.called = False
+
+        class Custom(dict):
+            def get(self, key, default=None):
+                test.called = True
+                super().get(key, default)
+
+        class Foo(object):
+            a = 3
+        foo = Foo()
+        foo.__dict__ = Custom()
+        self.assertEqual(inspect.getattr_static(foo, 'a'), 3)
+        self.assertFalse(test.called)
+
+    def test_metaclass_dict_as_property(self):
+        class Meta(type):
+            @property
+            def __dict__(self):
+                self.executed = True
+
+        class Thing(metaclass=Meta):
+            executed = False
+
+            def __init__(self):
+                self.spam = 42
+
+        instance = Thing()
+        self.assertEqual(inspect.getattr_static(instance, "spam"), 42)
+        self.assertFalse(Thing.executed)
 
 class TestGetGeneratorState(unittest.TestCase):
 

Misc/NEWS

@@ -40,6 +40,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11133: fix two cases where inspect.getattr_static can trigger code
+  execution. Patch by Daniel Urban.
+
 - Issue #11501: disutils.archive_utils.make_zipfile no longer fails if zlib is
   not installed. Instead, the zipfile.ZIP_STORED compression is used to create
   the ZipFile. Patch by Natalia B. Bidart.
@@ -48,7 +51,7 @@ Library
   encoding was not done if euc-jp or shift-jis was specified as the charset.
 
 - Issue #11500: Fixed a bug in the os x proxy bypass code for fully qualified
-  IP addresses in the proxy exception list. 
+  IP addresses in the proxy exception list.
 
 - Issue #11491: dbm.error is no longer raised when dbm.open is called with
   the "n" as the flag argument and the file exists. The behavior matches