upstream: Defend against user enumeration timing attacks. This

establishes a minimum time for each failed authentication attempt (5ms) and
adds a per-user constant derived from a host secret (0-4ms).  Based on work
by joona.kannisto at tut.fi, ok markus@ djm@.

OpenBSD-Commit-ID: b7845b355bb7381703339c8fb0e57e81a20ae5ca

1 files changed, 42 insertions, 1 deletions

diff --git a/auth2.c b/auth2.c
index e0034229a..2246e54d5 100644
--- a/auth2.c
+++ b/auth2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2.c,v 1.145 2018/03/03 03:15:51 djm Exp $ */
+/* $OpenBSD: auth2.c,v 1.146 2018/04/13 03:57:26 dtucker Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -57,6 +57,7 @@
 #endif
 #include "monitor_wrap.h"
 #include "ssherr.h"
+#include "digest.h"
 
 /* import */
 extern ServerOptions options;
@@ -210,6 +211,42 @@ input_service_request(int type, u_int32_t seq, struct ssh *ssh)
         return 0;
 }
 
+#define MIN_FAIL_DELAY_SECONDS 0.005
+static double
+user_specific_delay(const char *user)
+{
+        char b[512];
+        size_t len = ssh_digest_bytes(SSH_DIGEST_SHA512);
+        u_char *hash = xmalloc(len);
+        double delay;
+
+        (void)snprintf(b, sizeof b, "%llu%s", options.timing_secret, user);
+        if (ssh_digest_memory(SSH_DIGEST_SHA512, b, strlen(b), hash, len) != 0)
+                fatal("%s: ssh_digest_memory", __func__);
+        /* 0-4.2 ms of delay */
+        delay = (double)PEEK_U32(hash) / 1000 / 1000 / 1000 / 1000;
+        freezero(hash, len);
+        debug3("%s: user specific delay %0.3lfms", __func__, delay/1000);
+        return MIN_FAIL_DELAY_SECONDS + delay;
+}
+
+static void
+ensure_minimum_time_since(double start, double seconds)
+{
+        struct timespec ts;
+        double elapsed = monotime_double() - start, req = seconds, remain;
+
+        /* if we've already passed the requested time, scale up */
+        while ((remain = seconds - elapsed) < 0.0)
+                seconds *= 2;
+
+        ts.tv_sec = remain;
+        ts.tv_nsec = (remain - ts.tv_sec) * 1000000000;
+        debug3("%s: elapsed %0.3lfms, delaying %0.3lfms (requested %0.3lfms)",
+            __func__, elapsed*1000, remain*1000, req*1000);
+        nanosleep(&ts, NULL);
+}
+
 /*ARGSUSED*/
 static int
 input_userauth_request(int type, u_int32_t seq, struct ssh *ssh)
@@ -218,6 +255,7 @@ input_userauth_request(int type, u_int32_t seq, struct ssh *ssh)
         Authmethod *m = NULL;
         char *user, *service, *method, *style = NULL;
         int authenticated = 0;
+        double tstart = monotime_double();
 
         if (authctxt == NULL)
                 fatal("input_userauth_request: no authctxt");
@@ -286,6 +324,9 @@ input_userauth_request(int type, u_int32_t seq, struct ssh *ssh)
                 debug2("input_userauth_request: try method %s", method);
                 authenticated = m->userauth(ssh);
         }
+        if (!authctxt->authenticated)
+                ensure_minimum_time_since(tstart,
+                    user_specific_delay(authctxt->user));
         userauth_finish(ssh, authenticated, method, NULL);
 
         free(service);