[PATCH net 0/2] sctp: fix a vtag verification failure caused by stale INITs

Xin Long
2026-04-10T19:03:53Z

Similar to Scenario B in commit 8e56b063c865 ( netfilter: handle the
connecting collision properly in nf_conntrack_proto_sctp"):

Scenario B: INIT_ACK is delayed until the peer completes its own handshake

192.168.1.2 > 192.168.1.1: sctp (1) [INIT] [init tag: 3922216408]
192.168.1.1 > 192.168.1.2: sctp (1) [INIT] [init tag: 144230885]
192.168.1.2 > 192.168.1.1: sctp (1) [INIT ACK] [init tag: 3922216408]
192.168.1.1 > 192.168.1.2: sctp (1) [COOKIE ECHO]
192.168.1.2 > 192.168.1.1: sctp (1) [COOKIE ACK]
192.168.1.1 > 192.168.1.2: sctp (1) [INIT ACK] [init tag: 3914796021] *

There is another case:

Scenario F: INIT is delayed until the peer completes its own handshake

192.168.1.2 > 192.168.1.1: sctp (1) [INIT] [init tag: 3922216408]
(OVS upcall)
192.168.1.1 > 192.168.1.2: sctp (1) [INIT] [init tag: 144230885]
192.168.1.2 > 192.168.1.1: sctp (1) [INIT ACK] [init tag: 3922216408]
192.168.1.1 > 192.168.1.2: sctp (1) [COOKIE ECHO]
192.168.1.2 > 192.168.1.1: sctp (1) [COOKIE ACK]
192.168.1.2 > 192.168.1.1: sctp (1) [INIT] [init tag: 3922216408]
(delayed)
192.168.1.1 > 192.168.1.2: sctp (1) [INIT ACK] [init tag: 3914796021] *

In this case, the delayed INIT (e.g. due to OVS upcall) is recorded by
conntrack, which prevents vtag verification from dropping the unexpected
INIT-ACK in nf_conntrack_sctp_packet():

vtag = ct->proto.sctp.vtag[!dir];
if (!ct->proto.sctp.init[!dir] && vtag && vtag != ih->init_tag)
goto out_unlock;

This happens because ct->proto.sctp.init[!dir] is set by the delayed INIT,
even though it is stale.

Fix this in two parts:

- In netfilter: Do not record INITs whose init_tag matches the peer vtag,
as they carry no new handshake state in the 1st patch.

- In SCTP: Prevent endpoints from responding to such INITs with INIT-ACK,
ensuring correctness even when middleboxes lack the netfilter fix in
the 2nd patch.

A follow-up selftest for this scenario will be posted in a separate patch
by Yi Chen.

Xin Long (2):
netfilter: skip recording stale or retransmitted INIT
sctp: discard stale INIT after handshake completion

net/netfilter/nf_conntrack_proto_sctp.c | 10 +++++++---
net/sctp/sm_statefuns.c | 6 ++++++
2 files changed, 13 insertions(+), 3 deletions(-)

--
2.47.1