Subject: [PATCH net v2 0/2] sctp: fix a vtag verification failure caused by stale INITs

From: Xin Long <[email-masked]>
Date: Sun, 26 Apr 2026 10:46:39 -0400

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