Avoid possible timing attack in metadata api

Introduce a constant time comparison function to
nova utils for comparing authentication tokens.

Conflicts:
	nova/tests/test_utils.py
	nova/utils.py

Closes-bug: #1325128
Change-Id: I7374f2edc6f03c7da59cf73ae91a87147e53d0de
(cherry picked from commit 9f59ca751f)

nova/api/metadata/handler.py

@@ -31,6 +31,7 @@ from nova import exception
 from nova.openstack.common.gettextutils import _
 from nova.openstack.common import log as logging
 from nova.openstack.common import memorycache
+from nova import utils
 from nova import wsgi
 
 CACHE_EXPIRATION = 15  # in seconds
@@ -172,7 +173,7 @@ class MetadataRequestHandler(wsgi.Application):
             instance_id,
             hashlib.sha256).hexdigest()
 
-        if expected_signature != signature:
+        if not utils.constant_time_compare(expected_signature, signature):
             if instance_id:
                 LOG.warn(_('X-Instance-ID-Signature: %(signature)s does not '
                            'match the expected value: %(expected_signature)s '

nova/tests/test_utils.py

@@ -1083,3 +1083,10 @@ class GetImageFromSystemMetadataTestCase(test.NoDBTestCase):
 
         # Verify that the foo1 key has not been inherited
         self.assertTrue("foo1" not in image)
+
+
+class ConstantTimeCompareTestCase(test.NoDBTestCase):
+    def test_constant_time_compare(self):
+        self.assertTrue(utils.constant_time_compare("abcd1234", "abcd1234"))
+        self.assertFalse(utils.constant_time_compare("abcd1234", "a"))
+        self.assertFalse(utils.constant_time_compare("abcd1234", "ABCD234"))

nova/utils.py

@@ -23,6 +23,7 @@ import contextlib
 import datetime
 import functools
 import hashlib
+import hmac
 import inspect
 import os
 import pyclbr
@@ -1302,3 +1303,20 @@ def get_boolean(value):
         return value
     else:
         return strutils.bool_from_string(value)
+
+if hasattr(hmac, 'compare_digest'):
+    constant_time_compare = hmac.compare_digest
+else:
+    def constant_time_compare(first, second):
+        """Returns True if both string inputs are equal, otherwise False.
+
+        This function should take a constant amount of time regardless of
+        how many characters in the strings match.
+
+        """
+        if len(first) != len(second):
+            return False
+        result = 0
+        for x, y in zip(first, second):
+            result |= ord(x) ^ ord(y)
+        return result == 0