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

[Eric Dumazet]

On Wed, Aug 26, 2026 at 4:11 PM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx>
> ---

Nice!
So we had a bug for 10 years.
This is a testament to the fact that urgent mode is no longer used as
advised by RFC 6093.

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Thanks.