tcp: improve SEG.SEQ validation for RST segments

A RST segment can be sent in response to
(a) received segment or
(b) by the upper layer protocol.

The SEG.SEQ validation consists of two checks:
(1) the in-window check of SEG.SEQ and
(2) the exact match check of SEG.SEQ.

For the in-window check (1), the left edge of the window needs to be
based on tp->last_ack_sent to cover the delayed ACK case, whereas the
right edge needs to be based on tp->rcv_nxt + tp->rcv_wnd. This both
assumes that tp->rcv_wnd is not zero. For the special case of
tp->rcv_wnd being zero, add checks against tp->last_ack_sent for (a)
and on tp->rcv_nxt for (b). This applies to all TCP stacks.

When the exact match (2) of SEG.SEQ is performed, it should be based
on tp->last_ack_sent for (a) and on tp->rcv_nxt for (b). To cover both,
check for both. Add this only to the base stack, since the RACK and
BBR stacks already do this.

PR:			296594
Reviewed by:		rscheff
MFC after:		3 days
MFC to:			stable/14
MFC to:			stable/15
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D58594

sys/netinet/tcp_input.c

@@ -2125,20 +2125,26 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th,
 		 * - RST drops connection only if SEG.SEQ == RCV.NXT.
 		 * - If RST is in window, we send challenge ACK.
 		 *
-		 * Note: to take into account delayed ACKs, we should
-		 *   test against last_ack_sent instead of rcv_nxt.
+		 * Note 1: to take into account delayed ACKs, we should
+		 *   test against last_ack_sent in addition to rcv_nxt.
 		 * Note 2: we handle special case of closed window, not
 		 *   covered by the RFC.
+		 * Note 3 (XXXMT): check against rcv_adv instead of
+		 *   tp->rcv_nxt + tp->rcv_wnd.
 		 */
-		if ((SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
-		    SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) ||
-		    (tp->rcv_wnd == 0 && tp->last_ack_sent == th->th_seq)) {
+		if ((tp->rcv_wnd > 0 &&
+		     SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
+		     SEQ_LT(th->th_seq, tp->rcv_nxt + tp->rcv_wnd)) ||
+		    (tp->rcv_wnd == 0 &&
+		     (tp->last_ack_sent == th->th_seq ||
+		      tp->rcv_nxt == th->th_seq))) {
 			KASSERT(tp->t_state != TCPS_SYN_SENT,
 			    ("%s: TH_RST for TCPS_SYN_SENT th %p tp %p",
 			    __func__, th, tp));
 
 			if (V_tcp_insecure_rst ||
-			    tp->last_ack_sent == th->th_seq) {
+			    tp->last_ack_sent == th->th_seq ||
+			    tp->rcv_nxt == th->th_seq) {
 				TCPSTAT_INC(tcps_drops);
 				/* Drop the connection. */
 				switch (tp->t_state) {

sys/netinet/tcp_stacks/rack_bbr_common.c

@@ -697,18 +697,24 @@ ctf_process_rst(struct mbuf *m, struct tcphdr *th, struct socket *so,
 	/*
 	 * RFC5961 Section 3.2
 	 *
-	 * - RST drops connection only if SEG.SEQ == RCV.NXT. - If RST is in
-	 * window, we send challenge ACK.
+	 * - RST drops connection only if SEG.SEQ == RCV.NXT.
+	 * - If RST is in window, we send challenge ACK.
 	 *
-	 * Note: to take into account delayed ACKs, we should test against
-	 * last_ack_sent instead of rcv_nxt. Note 2: we handle special case
-	 * of closed window, not covered by the RFC.
+	 * Note 1: to take into account delayed ACKs, we should
+	 *   test against last_ack_sent in addition to rcv_nxt.
+	 * Note 2: we handle special case of closed window, not
+	 *   covered by the RFC.
+	 * Note 3 (XXXMT): check against rcv_adv instead of
+	 *   tp->rcv_nxt + tp->rcv_wnd.
 	 */
 	int dropped = 0;
 
-	if ((SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
-	    SEQ_LT(th->th_seq, tp->last_ack_sent + tp->rcv_wnd)) ||
-	    (tp->rcv_wnd == 0 && tp->last_ack_sent == th->th_seq)) {
+	if ((tp->rcv_wnd > 0 &&
+	     SEQ_GEQ(th->th_seq, tp->last_ack_sent) &&
+	     SEQ_LT(th->th_seq, tp->rcv_nxt + tp->rcv_wnd)) ||
+	    (tp->rcv_wnd == 0 &&
+	     (tp->last_ack_sent == th->th_seq ||
+	      tp->rcv_nxt == th->th_seq))) {
 		KASSERT(tp->t_state != TCPS_SYN_SENT,
 		    ("%s: TH_RST for TCPS_SYN_SENT th %p tp %p",
 		    __func__, th, tp));