Don't expose the uptime via the TCP timestamps.

The TCP client side or the TCP server side when not using SYN-cookies
used the uptime as the TCP timestamp value. This patch uses in all
cases an offset, which is the result of a keyed hash function taking
the source and destination addresses and port numbers into account.
The keyed hash function is the same a used for the initial TSN.

Reviewed by:		rrs@
MFC after:		1 month
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D16636

sys/netinet/tcp_subr.c

@@ -233,6 +233,9 @@ VNET_DEFINE(uma_zone_t, sack_hole_zone);
 VNET_DEFINE(struct hhook_head *, tcp_hhh[HHOOK_TCP_LAST+1]);
 #endif
 
+VNET_DEFINE_STATIC(u_char, ts_offset_secret[32]);
+#define	V_ts_offset_secret	VNET(ts_offset_secret)
+
 static int	tcp_default_fb_init(struct tcpcb *tp);
 static void	tcp_default_fb_fini(struct tcpcb *tp, int tcb_is_purged);
 static int	tcp_default_handoff_ok(struct tcpcb *tp);
@@ -1092,6 +1095,7 @@ tcp_init(void)
 	/* Initialize the TCP logging data. */
 	tcp_log_init();
 #endif
+	read_random(&V_ts_offset_secret, sizeof(V_ts_offset_secret));
 
 	if (tcp_soreceive_stream) {
 #ifdef INET
@@ -2603,6 +2607,40 @@ tcp6_ctlinput(int cmd, struct sockaddr *sa, void *d)
 }
 #endif /* INET6 */
 
+static uint32_t
+tcp_keyed_hash(struct in_conninfo *inc, u_char *key)
+{
+	MD5_CTX ctx;
+	uint32_t hash[4];
+
+	MD5Init(&ctx);
+	MD5Update(&ctx, &inc->inc_fport, sizeof(uint16_t));
+	MD5Update(&ctx, &inc->inc_lport, sizeof(uint16_t));
+	switch (inc->inc_flags & INC_ISIPV6) {
+#ifdef INET
+	case 0:
+		MD5Update(&ctx, &inc->inc_faddr, sizeof(struct in_addr));
+		MD5Update(&ctx, &inc->inc_laddr, sizeof(struct in_addr));
+		break;
+#endif
+#ifdef INET6
+	case INC_ISIPV6:
+		MD5Update(&ctx, &inc->inc6_faddr, sizeof(struct in6_addr));
+		MD5Update(&ctx, &inc->inc6_laddr, sizeof(struct in6_addr));
+		break;
+#endif
+	}
+	MD5Update(&ctx, key, 32);
+	MD5Final((unsigned char *)hash, &ctx);
+
+	return (hash[0]);
+}
+
+uint32_t
+tcp_new_ts_offset(struct in_conninfo *inc)
+{
+	return (tcp_keyed_hash(inc, V_ts_offset_secret));
+}
 
 /*
  * Following is where TCP initial sequence number generation occurs.
@@ -2644,7 +2682,7 @@ tcp6_ctlinput(int cmd, struct sockaddr *sa, void *d)
  * as reseeding should not be necessary.
  *
  * Locking of the global variables isn_secret, isn_last_reseed, isn_offset,
- * isn_offset_old, and isn_ctx is performed using the TCP pcbinfo lock.  In
+ * isn_offset_old, and isn_ctx is performed using the ISN lock.  In
  * general, this means holding an exclusive (write) lock.
  */
 
@@ -2665,15 +2703,11 @@ VNET_DEFINE_STATIC(u_int32_t, isn_offset_old);
 #define	V_isn_offset_old		VNET(isn_offset_old)
 
 tcp_seq
-tcp_new_isn(struct tcpcb *tp)
+tcp_new_isn(struct in_conninfo *inc)
 {
-	MD5_CTX isn_ctx;
-	u_int32_t md5_buffer[4];
 	tcp_seq new_isn;
 	u_int32_t projected_offset;
 
-	INP_WLOCK_ASSERT(tp->t_inpcb);
-
 	ISN_LOCK();
 	/* Seed if this is the first use, reseed if requested. */
 	if ((V_isn_last_reseed == 0) || ((V_tcp_isn_reseed_interval > 0) &&
@@ -2684,26 +2718,7 @@ tcp_new_isn(struct tcpcb *tp)
 	}
 
 	/* Compute the md5 hash and return the ISN. */
-	MD5Init(&isn_ctx);
+	new_isn = (tcp_seq)tcp_keyed_hash(inc, V_isn_secret);
-	MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_fport, sizeof(u_short));
-	MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_lport, sizeof(u_short));
-#ifdef INET6
-	if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) {
-		MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->in6p_faddr,
-			  sizeof(struct in6_addr));
-		MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->in6p_laddr,
-			  sizeof(struct in6_addr));
-	} else
-#endif
-	{
-		MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_faddr,
-			  sizeof(struct in_addr));
-		MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_laddr,
-			  sizeof(struct in_addr));
-	}
-	MD5Update(&isn_ctx, (u_char *) &V_isn_secret, sizeof(V_isn_secret));
-	MD5Final((u_char *) &md5_buffer, &isn_ctx);
-	new_isn = (tcp_seq) md5_buffer[0];
 	V_isn_offset += ISN_STATIC_INCREMENT +
 		(arc4random() & ISN_RANDOM_INCREMENT);
 	if (ticks != V_isn_last) {

sys/netinet/tcp_syncache.c

@@ -1497,6 +1497,7 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 		if (to->to_flags & TOF_TS) {
 			sc->sc_tsreflect = to->to_tsval;
 			sc->sc_flags |= SCF_TIMESTAMP;
+			sc->sc_tsoff = tcp_new_ts_offset(inc);
 		}
 		if (to->to_flags & TOF_SCALE) {
 			int wscale = 0;
@@ -2035,11 +2036,6 @@ syncookie_generate(struct syncache_head *sch, struct syncache *sc)
 	iss = hash & ~0xff;
 	iss |= cookie.cookie ^ (hash >> 24);
 
-	/* Randomize the timestamp. */
-	if (sc->sc_flags & SCF_TIMESTAMP) {
-		sc->sc_tsoff = arc4random() - tcp_ts_getticks();
-	}
-
 	TCPSTAT_INC(tcps_sc_sendcookie);
 	return (iss);
 }
@@ -2126,7 +2122,7 @@ syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
 	if (to->to_flags & TOF_TS) {
 		sc->sc_flags |= SCF_TIMESTAMP;
 		sc->sc_tsreflect = to->to_tsval;
-		sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
+		sc->sc_tsoff = tcp_new_ts_offset(inc);
 	}
 
 	if (to->to_flags & TOF_SIGNATURE)

sys/netinet/tcp_usrreq.c

@@ -1439,7 +1439,9 @@ tcp_connect(struct tcpcb *tp, struct sockaddr *nam, struct thread *td)
 	soisconnecting(so);
 	TCPSTAT_INC(tcps_connattempt);
 	tcp_state_change(tp, TCPS_SYN_SENT);
-	tp->iss = tcp_new_isn(tp);
+	tp->iss = tcp_new_isn(&inp->inp_inc);
+	if (tp->t_flags & TF_REQ_TSTMP)
+		tp->ts_offset = tcp_new_ts_offset(&inp->inp_inc);
 	tcp_sendseqinit(tp);
 
 	return 0;
@@ -1478,7 +1480,9 @@ tcp6_connect(struct tcpcb *tp, struct sockaddr *nam, struct thread *td)
 	soisconnecting(inp->inp_socket);
 	TCPSTAT_INC(tcps_connattempt);
 	tcp_state_change(tp, TCPS_SYN_SENT);
-	tp->iss = tcp_new_isn(tp);
+	tp->iss = tcp_new_isn(&inp->inp_inc);
+	if (tp->t_flags & TF_REQ_TSTMP)
+		tp->ts_offset = tcp_new_ts_offset(&inp->inp_inc);
 	tcp_sendseqinit(tp);
 
 	return 0;

sys/netinet/tcp_var.h

@@ -923,7 +923,9 @@ void	 tcp_hc_updatemtu(struct in_conninfo *, uint32_t);
 void	 tcp_hc_update(struct in_conninfo *, struct hc_metrics_lite *);
 
 extern	struct pr_usrreqs tcp_usrreqs;
-tcp_seq tcp_new_isn(struct tcpcb *);
+
+uint32_t tcp_new_ts_offset(struct in_conninfo *);
+tcp_seq	 tcp_new_isn(struct in_conninfo *);
 
 int	 tcp_sack_doack(struct tcpcb *, struct tcpopt *, tcp_seq);
 void	 tcp_update_sack_list(struct tcpcb *tp, tcp_seq rcv_laststart, tcp_seq rcv_lastend);