[PATCH net v2] calipso: keep BIG TCP payload_len in calipso_skbuff_setattr()

[Paul Moore]

On Sat, Oct 10, 2026 at 5:06 AM Wang Zhan <wang.zhan at smartx.com> wrote:
> On Fri, 09 Oct 2026 12:59:19 -0300 Joas Antonio dos Santos wrote:
> > calipso_skbuff_setattr() adjusts payload_len by the length of the
> > option it inserts or resizes:
> >
> >       payload = ntohs(ip6_hdr->payload_len);
> >       ip6_hdr->payload_len = htons(payload + len_delta);
> >
> > GRO can build BIG TCP packets larger than IPV6_MAXPLEN, and stores
> > payload_len == 0 for them; ipv6_payload_len() then falls back to
> > skb->len.  For such a packet forwarded to a destination mapped to
> > CALIPSO, the code above writes len_delta (or 65536 + len_delta when
> > shrinking) into payload_len, and ipv6_payload_len() no longer falls
> > back to skb->len.  nf_tables, conntrack and sch_cake rely on it.
> >
> > The same arithmetic also wraps when a packet grows past IPV6_MAXPLEN,
> > and a BIG TCP packet that shrinks below IPV6_MAXPLEN should get its
> > real length back.
> >
> > Set payload_len from the skb length with ipv6_set_payload_len(), which
> > stores 0 above IPV6_MAXPLEN and the real length otherwise, as
> > ipv6_gro_complete() does.  The network header was just reset to
> > skb->data, so skb->len - sizeof(*ip6_hdr) is the payload length.
> >
> > Tested on one kernel (arm64, QEMU) with SELinux permissive, a minimal
> > mdp-generated policy (network_peer_controls=1), "netlabelctl calipso
> > add pass doi:16" and a NetLabel mapping of fd00:2::/64 to CALIPSO
> > DOI 16.  calipso_skbuff_setattr() was called through
> > netlbl_skbuff_setattr() from a test-only debugfs hook (not part of
> > this patch) adding a level-only label; payload_len after the call:
> >
> >   case                         before fix   after fix   real length
> >   1000, unlabeled                    1016        1016          1016
> >   70000 GSO, payload_len 0             16           0         70016
> >   65530 GSO, grows past MAXPLEN        10           0         65546
> >   65540 GSO with a 24 byte hop
> >   header, shrinks below MAXPLEN     65528       65532         65532
> >   1000 with a 24 byte hop header      992         992           992
> >
> > (0 means ipv6_payload_len() returns the real length.)  On the forward
> > path, with a client injecting unlabeled UDP through a router to a
> > server, the router adds the CALIPSO option and the server delivers all
> > datagrams (Udp6InDatagrams 5, Ip6InTruncatedPkts 0) with this patch.
> >
> > Fixes: 81fbc812132c ("ipv6/gro: insert temporary HBH/jumbo header")
> > Suggested-by: Eric Dumazet <edumazet at kernel.org>
> > Suggested-by: Wang Zhan <wang.zhan at smartx.com>
> > Signed-off-by: Joas Antonio dos Santos <joasantonio108 at gmail.com>
> > Assisted-by: Claude:claude-opus-5-5
>
> Reviewed-by: Wang Zhan <wang.zhan at smartx.com>
>
> > @@ -1357,8 +1357,8 @@ static int calipso_skbuff_setattr(struct sk_buff *skb,
> >                       sizeof(*ip6_hdr) + start);
> >               skb_reset_network_header(skb);
> >               ip6_hdr = ipv6_hdr(skb);
> > -             payload = ntohs(ip6_hdr->payload_len);
> > -             ip6_hdr->payload_len = htons(payload + len_delta);
> > +             /* 0 above IPV6_MAXPLEN (BIG TCP), the real length otherwise */
>
> nit: consider dropping the comment, it just repeats what
> ipv6_set_payload_len() does.

Agree with the nit, but otherwise this looks good to me.  Thanks to
Joas Antonio dos Santos for identifying the problem and working on the
fix as well as everyone else who took the time to review and comment;
the help is always appreciated!

Acked-by: Paul Moore <paul at paul-moore.com>

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