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

[Simon Horman <horms@kernel.org>]

On Sun, Apr 26, 2026 at 10:46:39AM -0400, Xin Long wrote:
> 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.

Hi Xin,

FTR: There is an AI generated review of this patchset available on
sashiko.dev. I have looked over this and I do not believe the feedback
there should block progress of this patchset.