[PATCH net 1/2] tcp: fix corruption of urgent data on multi-segment retransmit

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On the normal xmit path, while in urgent mode we refuse to build a
multi-segment TSO packet, so every segment gets its own urg_ptr:

	/* tcp_write_xmit() */
	limit = mss_now;
	if (tso_segs > 1 && !tcp_urg_mode(tp))
		limit = tcp_mss_split_point(...);

The retransmit path has no such guard. __tcp_retransmit_skb() builds a
segs > 1 skb and hands it to the GSO layer, which only advances th->seq
per segment and copies urg_ptr verbatim:

	/* __tcp_retransmit_skb() */
	len = cur_mss * segs;		/* segs > 1, no urg_mode check */
	...
	/* tcp_gso_segment(): bumps seq only, urg_ptr is copied */

urg_ptr is an offset from the segment's own seq, so a copied value points
at a different place on each segment. The receiver rebuilds the absolute
urgent seq as seg.seq + urg_ptr, so it walks a moving urgent point instead
of the one OOB byte:

	seg1  seq 1     urg_ptr 5001 -> urgent @ 5001   (ok)
	seg2  seq 1001  urg_ptr 5001 -> urgent @ 6001   (wrong, +MSS)
	seg3  seq 2001  urg_ptr 5001 -> urgent @ 7001   (wrong, +2*MSS)

The real OOB byte is never pointed at, so the receiver stops splicing it
out and delivers it as normal in-band data, corrupting the stream.

Guard the retransmit length like the xmit path: keep segs = 1 while in
urgent mode.

Fixes: 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit time")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/ipv4/tcp_output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index fcaa04e65189..86e255794027 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3603,7 +3603,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		avail_wnd = cur_mss;
 	}
 
-	len = cur_mss * segs;
+	len = cur_mss * (tcp_urg_mode(tp) ? 1 : segs);
 	if (len > avail_wnd) {
 		len = rounddown(avail_wnd, cur_mss);
 		if (!len)
-- 
2.43.0