tcp: When we have an out-of-order FIN we do want to strip off the FIN bit.

The last set of commits fixed both a panic (in rack) and an ACK-war (in freebsd and bbr).
However there was a missing case, i.e. where we get an out-of-order FIN by itself.
In such a case we don't want to leave the FIN bit set, otherwise we will do the
wrong thing and ack the FIN incorrectly. Instead we need to go through the
tcp_reasm() code and that way the FIN will be stripped and all will be well.

Reviewed by: mtuexen,rscheff
Sponsored by: Netflix Inc
Differential Revision:	https://reviews.freebsd.org/D30497

sys/netinet/tcp_input.c

@@ -3191,7 +3191,12 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 			 * when trimming from the head.
 			 */
 			tcp_seq temp = save_start;
-			if (tlen) {
+			if (tlen || (th->th_seq != tp->rcv_nxt)) {
+				/*
+				 * We add the th_seq != rcv_nxt to
+				 * catch the case of a stand alone out
+				 * of order FIN.
+				 */
 				thflags = tcp_reass(tp, th, &temp, &tlen, m);
 				tp->t_flags |= TF_ACKNOW;
 			}

sys/netinet/tcp_stacks/bbr.c

@@ -8320,7 +8320,12 @@ bbr_process_data(struct mbuf *m, struct tcphdr *th, struct socket *so,
 			 * trimming from the head.
 			 */
 			tcp_seq temp = save_start;
-			if (tlen) {
+			if (tlen || (th->th_seq != tp->rcv_nxt)) {
+				/*
+				 * We add the th_seq != rcv_nxt to
+				 * catch the case of a stand alone out
+				 * of order FIN.
+				 */
 				thflags = tcp_reass(tp, th, &temp, &tlen, m);
 				tp->t_flags |= TF_ACKNOW;
 			}

sys/netinet/tcp_stacks/rack.c

@@ -10235,7 +10235,12 @@ rack_process_data(struct mbuf *m, struct tcphdr *th, struct socket *so,
 			 * trimming from the head.
 			 */
 			tcp_seq temp = save_start;
-			if (tlen) {
+			if (tlen || (th->th_seq != tp->rcv_nxt)) {
+				/*
+				 * We add the th_seq != rcv_nxt to
+				 * catch the case of a stand alone out
+				 * of order FIN.
+				 */
 				thflags = tcp_reass(tp, th, &temp, &tlen, m);
 				tp->t_flags |= TF_ACKNOW;
 			}