git: 2a38a33b221e - stable/14 - tcp: fix the test that a duplicate ACK has no data

The branch stable/14 has been updated by tuexen:

URL: https://cgit.FreeBSD.org/src/commit/?id=2a38a33b221ea96389e95eb7fa5d8ac4c6664a9e

commit 2a38a33b221ea96389e95eb7fa5d8ac4c6664a9e
Author:     Michael Tuexen <tuexen@FreeBSD.org>
AuthorDate: 2025-07-19 13:03:22 +0000
Commit:     Michael Tuexen <tuexen@FreeBSD.org>
CommitDate: 2025-09-05 06:49:07 +0000

    tcp: fix the test that a duplicate ACK has no data
    
    When processing a TCP segment, data is removed from the head or
    the tail. The test whether a segment has no data on it should
    depend on the TCP segment before the removal. Without this,
    received segments might trigger a fast retransmit even when they
    should not.
    
    Reported by:            syzbot+fc97a2b5a0f7ea161161@syzkaller.appspotmail.com
    Reviewed by:            Peter Lei
    Sponsored by:           Netflix, Inc.
    Differential Revision:  https://reviews.freebsd.org/D51425
    
    (cherry picked from commit e46c15a0f848174f58bb08fed8e173a76b4c473a)
---
 sys/netinet/tcp_input.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index fb0c110d460d..f4d061557e88 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -1541,7 +1541,9 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th,
 	struct tcpopt to;
 	int tfo_syn;
 	u_int maxseg;
+	bool no_data;
 
+	no_data = (tlen == 0);
 	thflags = tcp_get_flags(th);
 	tp->sackhint.last_sack_ack = 0;
 	sack_changed = SACK_NOCHANGE;
@@ -1769,7 +1771,7 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th,
 			tp->ts_recent = to.to_tsval;
 		}
 
-		if (tlen == 0) {
+		if (no_data) {
 			if (SEQ_GT(th->th_ack, tp->snd_una) &&
 			    SEQ_LEQ(th->th_ack, tp->snd_max) &&
 			    !IN_RECOVERY(tp->t_flags) &&
@@ -2570,7 +2572,7 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th,
 
 		if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
 			maxseg = tcp_maxseg(tp);
-			if (tlen == 0 &&
+			if (no_data &&
 			    (tiwin == tp->snd_wnd ||
 			    (tp->t_flags & TF_SACK_PERMIT))) {
 				/*
@@ -3104,8 +3106,7 @@ step6:
 	    (tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
 	     (tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd))))) {
 		/* keep track of pure window updates */
-		if (tlen == 0 &&
-		    tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
+		if (no_data && tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)
 			TCPSTAT_INC(tcps_rcvwinupd);
 		tp->snd_wnd = tiwin;
 		tp->snd_wl1 = th->th_seq;