In the Linux kernel, the following vulnerability has been resolved: tcp: fix corruption of urgent data on multi-segment retransmit On the

In the Linux kernel, the following vulnerability has been resolved:

tcp: fix corruption of urgent data on multi-segment retransmit

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:

 1/* tcp_write_xmit() */
 2limit = mss_now;
 3if (tso_segs > 1 && !tcp_urg_mode(tp))
 4	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:

 1/* __tcp_retransmit_skb() */
 2len = cur_mss * segs;		/* segs > 1, no urg_mode check */
 3...
 4/* 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:

 1seg1  seq 1     urg_ptr 5001 -> urgent @ 5001   (ok)
 2seg2  seq 1001  urg_ptr 5001 -> urgent @ 6001   (wrong, +MSS)
 3seg3  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.