tcp: fix sequence numbers for repaired sockets re-using TIME-WAIT sockets

This patch fixes a bug where the sequence numbers of a socket created using
TCP repair functionality are lower than set after connect is called.
This occurs when the repair socket overlaps with a TIME-WAIT socket and
triggers the re-use code. The amount lower is equal to the number of times
that a particular IP/port set is re-used and then put back into TIME-WAIT.
Re-using the first time the sequence number is 1 lower, closing that socket
and then re-opening (with repair) a new socket with the same addresses/ports
puts the sequence number 2 lower than set via setsockopt. The third time is
3 lower, etc. I have not tested what the limit of this acrewal is, if any.

The fix is, if a socket is in repair mode, to respect the already set
sequence number and timestamp when it would have already re-used the
TIME-WAIT socket.
Signed-off-by: Stefan Baranoff <sbaranoff@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -156,11 +156,24 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp)
 	 */
 	if (tcptw->tw_ts_recent_stamp &&
 	    (!twp || (reuse && get_seconds() - tcptw->tw_ts_recent_stamp > 1))) {
-		tp->write_seq = tcptw->tw_snd_nxt + 65535 + 2;
-		if (tp->write_seq == 0)
-			tp->write_seq = 1;
-		tp->rx_opt.ts_recent	   = tcptw->tw_ts_recent;
-		tp->rx_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
+		/* In case of repair and re-using TIME-WAIT sockets we still
+		 * want to be sure that it is safe as above but honor the
+		 * sequence numbers and time stamps set as part of the repair
+		 * process.
+		 *
+		 * Without this check re-using a TIME-WAIT socket with TCP
+		 * repair would accumulate a -1 on the repair assigned
+		 * sequence number. The first time it is reused the sequence
+		 * is -1, the second time -2, etc. This fixes that issue
+		 * without appearing to create any others.
+		 */
+		if (likely(!tp->repair)) {
+			tp->write_seq = tcptw->tw_snd_nxt + 65535 + 2;
+			if (tp->write_seq == 0)
+				tp->write_seq = 1;
+			tp->rx_opt.ts_recent	   = tcptw->tw_ts_recent;
+			tp->rx_opt.ts_recent_stamp = tcptw->tw_ts_recent_stamp;
+		}
 		sock_hold(sktw);
 		return 1;
 	}