Use constant time string comparisons for auth.

Fix bug 942644.

Use constant time string comparisons when doing authentication to help
guard against timing attacks.

Change-Id: Iaaefb13f7618b06834630d9ccb97aff056b4bf4c

nova/auth/manager.py

@@ -328,7 +328,7 @@ class AuthManager(object):
             LOG.debug(_('user.secret: %s'), user.secret)
             LOG.debug(_('expected_signature: %s'), expected_signature)
             LOG.debug(_('signature: %s'), signature)
-            if signature != expected_signature:
+            if not utils.strcmp_const_time(signature, expected_signature):
                 LOG.audit(_("Invalid signature for user %s"), user.name)
                 raise exception.InvalidSignature(signature=signature,
                                                  user=user)
@@ -340,7 +340,7 @@ class AuthManager(object):
             LOG.debug(_('user.secret: %s'), user.secret)
             LOG.debug(_('expected_signature: %s'), expected_signature)
             LOG.debug(_('signature: %s'), signature)
-            if signature != expected_signature:
+            if not utils.strcmp_const_time(signature, expected_signature):
                 (addr_str, port_str) = utils.parse_server_string(server_string)
                 # If the given server_string contains port num, try without it.
                 if port_str != '':
@@ -349,7 +349,7 @@ class AuthManager(object):
                                                        addr_str, path)
                     LOG.debug(_('host_only_signature: %s'),
                               host_only_signature)
-                    if signature == host_only_signature:
+                    if utils.strcmp_const_time(signature, host_only_signature):
                         return (user, project)
                 LOG.audit(_("Invalid signature for user %s"), user.name)
                 raise exception.InvalidSignature(signature=signature,

nova/utils.py

@@ -1560,3 +1560,23 @@ def tempdir(**kwargs):
             shutil.rmtree(tmpdir)
         except OSError, e:
             LOG.debug(_('Could not remove tmpdir: %s'), str(e))
+
+
+def strcmp_const_time(s1, s2):
+    """Constant-time string comparison.
+
+    :params s1: the first string
+    :params s2: the second string
+
+    :return: True if the strings are equal.
+
+    This function takes two strings and compares them.  It is intended to be
+    used when doing a comparison for authentication purposes to help guard
+    against timing attacks.
+    """
+    if len(s1) != len(s2):
+        return False
+    result = 0
+    for (a, b) in zip(s1, s2):
+        result |= ord(a) ^ ord(b)
+    return result == 0