Avoid possible timing attack in metadata api

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

Change-Id: I7374f2edc6f03c7da59cf73ae91a87147e53d0de
Closes-bug: #1325128

nova/api/metadata/handler.py

@@ -30,6 +30,7 @@ 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
@@ -175,7 +176,7 @@ 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

@@ -962,3 +962,10 @@ class VersionTestCase(test.NoDBTestCase):
 
     def test_convert_version_to_tuple(self):
         self.assertEqual(utils.convert_version_to_tuple('6.7.0'), (6, 7, 0))
+
+
+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

@@ -21,6 +21,7 @@ import contextlib
 import datetime
 import functools
 import hashlib
+import hmac
 import inspect
 import os
 import pyclbr
@@ -1144,3 +1145,20 @@ def get_image_from_system_metadata(system_meta):
 def get_hash_str(base_str):
     """returns string that represents hash of base_str (in hex format)."""
     return hashlib.md5(base_str).hexdigest()
+
+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