Ticket bz1358565 - clear and unsalted password types are vulnerable to timing attack

Bug Description:  Clear and unsalted password types were vulnerable to a timing
attack. This is due to the use of memcmp and strcmp in their comparison.

Fix Description:  Add a constant time memcmp function, that does not shortcircuit.
Change all password comparison to use the constant time check. For the clear
scheme, alter the way we do the check to prevent length disclosure timing
attacks.

This resolves CVE-2016-5405

https://bugzilla.redhat.com/show_bug.cgi?id=1358565

https://access.redhat.com/security/cve/CVE-2016-5405

Author: wibrown

Review by: nhosoi (Thanks!)

(cherry picked from commit 9dcaa4a0c866d8696e0a2616ccf962af2833f0b8)
(cherry picked from commit f0e03b5a51972a125fe78f448d1f68e288782d1e)

dirsrvtests/tests/suites/password/pwd_algo_test.py

@@ -0,0 +1,143 @@
+import os
+import sys
+import time
+import ldap
+import logging
+import pytest
+from lib389 import DirSrv, Entry, tools, tasks
+from lib389.tools import DirSrvTools
+from lib389._constants import *
+from lib389.properties import *
+from lib389.tasks import *
+from lib389.utils import *
+
+DEBUGGING = True
+USER_DN = 'uid=user,ou=People,%s' % DEFAULT_SUFFIX
+
+if DEBUGGING:
+    logging.getLogger(__name__).setLevel(logging.DEBUG)
+else:
+    logging.getLogger(__name__).setLevel(logging.INFO)
+
+
+log = logging.getLogger(__name__)
+
+
+class TopologyStandalone(object):
+    """The DS Topology Class"""
+    def __init__(self, standalone):
+        """Init"""
+        standalone.open()
+        self.standalone = standalone
+
+
[email protected](scope="module")
+def topology(request):
+    """Create DS Deployment"""
+
+    # Creating standalone instance ...
+    if DEBUGGING:
+        standalone = DirSrv(verbose=True)
+    else:
+        standalone = DirSrv(verbose=False)
+    args_instance[SER_HOST] = HOST_STANDALONE
+    args_instance[SER_PORT] = PORT_STANDALONE
+    args_instance[SER_SERVERID_PROP] = SERVERID_STANDALONE
+    args_instance[SER_CREATION_SUFFIX] = DEFAULT_SUFFIX
+    args_standalone = args_instance.copy()
+    standalone.allocate(args_standalone)
+    instance_standalone = standalone.exists()
+    if instance_standalone:
+        standalone.delete()
+    standalone.create()
+    standalone.open()
+
+    def fin():
+        """If we are debugging just stop the instances, otherwise remove
+        them
+        """
+        if DEBUGGING:
+            standalone.stop()
+        else:
+            standalone.delete()
+
+    request.addfinalizer(fin)
+
+    # Clear out the tmp dir
+    standalone.clearTmpDir(__file__)
+
+    return TopologyStandalone(standalone)
+
+def _test_bind(inst, password):
+    result = True
+    userconn = ldap.initialize("ldap://%s:%s" % (HOST_STANDALONE, PORT_STANDALONE))
+    try:
+        userconn.simple_bind_s(USER_DN, password)
+        userconn.unbind_s()
+    except ldap.INVALID_CREDENTIALS:
+        result = False
+    return result
+
+def _test_algo(inst, algo_name):
+    inst.config.set('passwordStorageScheme', algo_name)
+
+    if DEBUGGING:
+        print('Testing %s', algo_name)
+
+    # Create the user with a password
+    inst.add_s(Entry((
+                USER_DN, {
+                    'objectClass': 'top account simplesecurityobject'.split(),
+                     'uid': 'user',
+                     'userpassword': 'Secret123'
+                })))
+
+    # Make sure when we read the userPassword field, it is the correct ALGO
+    pw_field = inst.search_s(USER_DN, ldap.SCOPE_BASE, '(objectClass=*)', ['userPassword']  )[0]
+
+    if DEBUGGING:
+        print(pw_field.getValue('userPassword'))
+
+    if algo_name != 'CLEAR':
+        assert(algo_name.lower() in pw_field.getValue('userPassword').lower())
+    # Now make sure a bind works
+    assert(_test_bind(inst, 'Secret123'))
+    # Bind with a wrong shorter password, should fail
+    assert(not _test_bind(inst, 'Wrong'))
+    # Bind with a wrong longer password, should fail
+    assert(not _test_bind(inst, 'This is even more wrong'))
+    # Bind with a wrong exact length password.
+    assert(not _test_bind(inst, 'Alsowrong'))
+    # Bind with a subset password, should fail
+    assert(not _test_bind(inst, 'Secret'))
+    if algo_name != 'CRYPT':
+        # Bind with a subset password that is 1 char shorter, to detect off by 1 in clear
+        assert(not _test_bind(inst, 'Secret12'))
+        # Bind with a superset password, should fail
+        assert(not _test_bind(inst, 'Secret123456'))
+    # Delete the user
+    inst.delete_s(USER_DN)
+    # done!
+
+def test_pwd_algo_test(topology):
+    """
+    Assert that all of our password algorithms correctly PASS and FAIL varying
+    password conditions.
+
+    """
+    if DEBUGGING:
+        # Add debugging steps(if any)...
+        pass
+
+    for algo in ('CLEAR', 'CRYPT', 'MD5', 'SHA', 'SHA256', 'SHA384', 'SHA512', 'SMD5', 'SSHA', 'SSHA256', 'SSHA384', 'SSHA512'):
+        _test_algo(topology.standalone, algo)
+
+    log.info('Test PASSED')
+
+
+if __name__ == '__main__':
+    # Run isolated
+    # -s for DEBUG mode
+    CURRENT_FILE = os.path.realpath(__file__)
+    pytest.main("-s %s" % CURRENT_FILE)
+

ldap/servers/plugins/pwdstorage/clear_pwd.c

@@ -26,6 +26,7 @@ int
 clear_pw_cmp( const char *userpwd, const char *dbpwd )
 {
     int result = 0;
+    int len = 0;
     int len_user = strlen(userpwd);
     int len_dbp = strlen(dbpwd);
     if ( len_user != len_dbp ) {

ldap/servers/plugins/pwdstorage/crypt_pwd.c

@@ -54,7 +54,7 @@ crypt_pw_cmp( const char *userpwd, const char *dbpwd )
     /* we use salt (first 2 chars) of encoded password in call to crypt() */
     cp = crypt( userpwd, dbpwd );
     if (cp) {
-       rc= strcmp( dbpwd, cp);
+       rc= slapi_ct_memcmp( dbpwd, cp, strlen(dbpwd));
     } else {
        rc = -1;
     }

ldap/servers/plugins/pwdstorage/md5_pwd.c

@@ -57,7 +57,7 @@ md5_pw_cmp( const char *userpwd, const char *dbpwd )
    bver = NSSBase64_EncodeItem(NULL, (char *)b2a_out, sizeof b2a_out, &binary_item);
    /* bver points to b2a_out upon success */
    if (bver) {
-	   rc = strcmp(bver,dbpwd);
+	   rc = slapi_ct_memcmp(bver,dbpwd, strlen(dbpwd));
    } else {
 	   slapi_log_err(SLAPI_LOG_PLUGIN, MD5_SUBSYSTEM_NAME,
 					   "Could not base64 encode hashed value for password compare");

ldap/servers/plugins/pwdstorage/ns-mta-md5_pwd.c

@@ -84,6 +84,7 @@ ns_mta_md5_pw_cmp(const char * clear, const char *mangled)
 
   mta_hash[32] = mta_salt[32] = 0;
 
+  /* This is salted, so we don't need to change it for constant time */
   return( strcmp(mta_hash,ns_mta_hash_alg(buffer,mta_salt,clear)));
 }
 

ldap/servers/plugins/pwdstorage/sha_pwd.c

@@ -120,13 +120,16 @@ sha_pw_cmp (const char *userpwd, const char *dbpwd, unsigned int shaLen )
     }
 
     /* the proof is in the comparison... */
-    result = ( hash_len >= shaLen ) ?
-        ( memcmp( userhash, dbhash, shaLen ) ) : /* include salt */
-        ( memcmp( userhash, dbhash + OLD_SALT_LENGTH,
-                  hash_len - OLD_SALT_LENGTH ) ); /* exclude salt */
+    if ( hash_len >= shaLen ) {
+        result = slapi_ct_memcmp( userhash, dbhash, shaLen );
+    } else {
+        result = slapi_ct_memcmp( userhash, dbhash + OLD_SALT_LENGTH, hash_len - OLD_SALT_LENGTH );
+    }
 
-    loser:
-    if ( dbhash && dbhash != quick_dbhash ) slapi_ch_free_string( &dbhash );
+loser:
+    if ( dbhash && dbhash != quick_dbhash ) {
+        slapi_ch_free_string( &dbhash );
+    }
     return result;
 }
 

ldap/servers/plugins/pwdstorage/smd5_pwd.c

@@ -80,7 +80,7 @@ smd5_pw_cmp( const char *userpwd, const char *dbpwd )
    PK11_DestroyContext(ctx, 1);
 
    /* Compare everything up to the salt. */
-   rc = memcmp( userhash, dbhash, MD5_LENGTH );
+   rc = slapi_ct_memcmp( userhash, dbhash, MD5_LENGTH );
 
 loser:
    if ( dbhash && dbhash != quick_dbhash ) slapi_ch_free_string( (char **)&dbhash );

ldap/servers/slapd/ch_malloc.c

@@ -340,3 +340,25 @@ slapi_ch_smprintf(const char *fmt, ...)
     return p;
 }
 #endif
+
+/* Constant time memcmp. Does not shortcircuit on failure! */
+/* This relies on p1 and p2 both being size at least n! */
+int
+slapi_ct_memcmp( const void *p1, const void *p2, size_t n)
+{
+    int result = 0;
+    const unsigned char *_p1 = (const unsigned char *)p1;
+    const unsigned char *_p2 = (const unsigned char *)p2;
+
+    if (_p1 == NULL || _p2 == NULL) {
+        return 2;
+    }
+
+    for (size_t i = 0; i < n; i++) {
+        if (_p1[i] ^ _p2[i]) {
+            result = 1;
+        }
+    }
+    return result;
+}
+

ldap/servers/slapd/slapi-plugin.h

@@ -5837,6 +5837,22 @@ char * slapi_ch_smprintf(const char *fmt, ...)
 #else
         ;
 #endif
+/**
+ * slapi_ct_memcmp is a constant time memory comparison function. This is for
+ * use with password hashes and other locations which could lead to a timing
+ * attack due to early shortcut returns. This function *does not* shortcircuit
+ * during the comparison, always checking every byte regardless if it has already
+ * found that the memory does not match.
+ *
+ * WARNING! p1 and p2 must both reference content that is at least of size 'n'.
+ * Else this function may over-run (And will certainly fail).
+ *
+ * \param p1 pointer to first value to check.
+ * \param p2 pointer to second value to check.
+ * \param n length in bytes of the content of p1 AND p2.
+ * \return 0 on match. 1 on non-match. 2 on presence of NULL pointer in p1 or p2.
+ */
+int slapi_ct_memcmp( const void *p1, const void *p2, size_t n);
 
 /*
  * syntax plugin routines