Fixed #15716 - Authentication backends can short-circuit authorization.

Authorization backends can now raise PermissionDenied in "has_perm"
and "has_module_perms" to short-circuit authorization process.

django/contrib/auth/models.py

 from __future__ import unicode_literals
 
+from django.core.exceptions import PermissionDenied
 from django.core.mail import send_mail
 from django.core import validators
 from django.db import models
@@ -267,18 +268,32 @@ def _user_get_all_permissions(user, obj):
 
 
 def _user_has_perm(user, perm, obj):
+    """
+    A backend can raise `PermissionDenied` to short-circuit permission checking.
+    """
     for backend in auth.get_backends():
-        if hasattr(backend, "has_perm"):
+        if not hasattr(backend, 'has_perm'):
+            continue
+        try:
             if backend.has_perm(user, perm, obj):
                 return True
+        except PermissionDenied:
+            return False
     return False
 
 
 def _user_has_module_perms(user, app_label):
+    """
+    A backend can raise `PermissionDenied` to short-circuit permission checking.
+    """
     for backend in auth.get_backends():
-        if hasattr(backend, "has_module_perms"):
+        if not hasattr(backend, 'has_module_perms'):
+            continue
+        try:
             if backend.has_module_perms(user, app_label):
                 return True
+        except PermissionDenied:
+            return False
     return False
 
 

django/contrib/auth/tests/test_auth_backends.py

@@ -398,7 +398,7 @@ class InActiveUserBackendTest(TestCase):
 
 class PermissionDeniedBackend(object):
     """
-    Always raises PermissionDenied.
+    Always raises PermissionDenied in `authenticate`, `has_perm` and `has_module_perms`.
     """
     supports_object_permissions = True
     supports_anonymous_user = True
@@ -407,6 +407,12 @@ class PermissionDeniedBackend(object):
     def authenticate(self, username=None, password=None):
         raise PermissionDenied
 
+    def has_perm(self, user_obj, perm, obj=None):
+        raise PermissionDenied
+
+    def has_module_perms(self, user_obj, app_label):
+        raise PermissionDenied
+
 
 @skipIfCustomUser
 class PermissionDeniedBackendTest(TestCase):
@@ -430,6 +436,26 @@ class PermissionDeniedBackendTest(TestCase):
     def test_authenticates(self):
         self.assertEqual(authenticate(username='test', password='test'), self.user1)
 
+    @override_settings(AUTHENTICATION_BACKENDS=(backend, ) +
+            tuple(settings.AUTHENTICATION_BACKENDS))
+    def test_has_perm_denied(self):
+        content_type = ContentType.objects.get_for_model(Group)
+        perm = Permission.objects.create(name='test', content_type=content_type, codename='test')
+        self.user1.user_permissions.add(perm)
+
+        self.assertIs(self.user1.has_perm('auth.test'), False)
+        self.assertIs(self.user1.has_module_perms('auth'), False)
+
+    @override_settings(AUTHENTICATION_BACKENDS=tuple(
+        settings.AUTHENTICATION_BACKENDS) + (backend, ))
+    def test_has_perm(self):
+        content_type = ContentType.objects.get_for_model(Group)
+        perm = Permission.objects.create(name='test', content_type=content_type, codename='test')
+        self.user1.user_permissions.add(perm)
+
+        self.assertIs(self.user1.has_perm('auth.test'), True)
+        self.assertIs(self.user1.has_module_perms('auth'), True)
+
 
 class NewModelBackend(ModelBackend):
     pass

docs/releases/1.8.txt

@@ -36,7 +36,11 @@ Minor features
 :mod:`django.contrib.auth`
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-* ...
+* Authorization backends can now raise
+  :class:`~django.core.exceptions.PermissionDenied` in
+  :meth:`~django.contrib.auth.models.User.has_perm`
+  and :meth:`~django.contrib.auth.models.User.has_module_perms`
+  to short-circuit permission checking.
 
 :mod:`django.contrib.formtools`
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

docs/topics/auth/customizing.txt

@@ -180,6 +180,14 @@ The permissions given to the user will be the superset of all permissions
 returned by all backends. That is, Django grants a permission to a user that
 any one backend grants.
 
+.. versionadded:: 1.8
+
+    If a backend raises a :class:`~django.core.exceptions.PermissionDenied`
+    exception in :meth:`~django.contrib.auth.models.User.has_perm()` or
+    :meth:`~django.contrib.auth.models.User.has_module_perms()`,
+    the authorization will immediately fail and Django
+    won't check the backends that follow.
+
 The simple backend above could implement permissions for the magic admin
 fairly simply::