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

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

[Xin Long]

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

[PATCH net 1/2] netfilter: skip recording stale or retransmitted INIT

[Xin Long]

An INIT whose init_tag matches the peer's vtag does not provide new state
information. It indicates either:

- a stale INIT (after INIT-ACK has already been seen on the same side), or
- a retransmitted INIT (after INIT has already been recorded on the same
  side).

In both cases, the INIT must not update ct->proto.sctp.init[] state, since
it does not advance the handshake tracking and may otherwise corrupt
INIT/INIT-ACK validation logic.

Allow INIT processing only when the conntrack entry is newly created
(SCTP_CONNTRACK_NONE), or when the init_tag differs from the stored peer
vtag.

Note it skips the check for the ct with old_state SCTP_CONNTRACK_NONE in
nf_conntrack_sctp_packet(), as it is just created in sctp_new() where it
set ct->proto.sctp.vtag[IP_CT_DIR_REPLY] = ih->init_tag.

Fixes: 9fb9cbb1082d ("[NETFILTER]: Add nf_conntrack subsystem.")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/netfilter/nf_conntrack_proto_sctp.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/netfilter/nf_conntrack_proto_sctp.c b/net/netfilter/nf_conntrack_proto_sctp.c
index 645d2c43ebf7..7e10fa65cbdd 100644
--- a/net/netfilter/nf_conntrack_proto_sctp.c
+++ b/net/netfilter/nf_conntrack_proto_sctp.c
@@ -466,9 +466,13 @@ int nf_conntrack_sctp_packet(struct nf_conn *ct,
 			if (!ih)
 				goto out_unlock;
 
-			if (ct->proto.sctp.init[dir] && ct->proto.sctp.init[!dir])
-				ct->proto.sctp.init[!dir] = 0;
-			ct->proto.sctp.init[dir] = 1;
+			/* Do not record INIT matching peer vtag (stale or retransmitted INIT). */
+			if (old_state == SCTP_CONNTRACK_NONE ||
+			    ct->proto.sctp.vtag[!dir] != ih->init_tag) {
+				if (ct->proto.sctp.init[dir] && ct->proto.sctp.init[!dir])
+					ct->proto.sctp.init[!dir] = 0;
+				ct->proto.sctp.init[dir] = 1;
+			}
 
 			pr_debug("Setting vtag %x for dir %d\n", ih->init_tag, !dir);
 			ct->proto.sctp.vtag[!dir] = ih->init_tag;
-- 
2.47.1

[PATCH net 2/2] sctp: discard stale INIT after handshake completion

[Xin Long]

After an association reaches ESTABLISHED, the peer’s init_tag is already
known from the handshake. Any subsequent INIT with the same init_tag is
not a valid restart, but a delayed or duplicate INIT.

Drop such INIT chunks in sctp_sf_do_unexpected_init() instead of
processing them as new association attempts.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/sctp/sm_statefuns.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
index 7b823d759141..3bec026ecbc0 100644
--- a/net/sctp/sm_statefuns.c
+++ b/net/sctp/sm_statefuns.c
@@ -1556,6 +1556,12 @@ static enum sctp_disposition sctp_sf_do_unexpected_init(
 	/* Tag the variable length parameters.  */
 	chunk->param_hdr.v = skb_pull(chunk->skb, sizeof(struct sctp_inithdr));
 
+	if (asoc->state >= SCTP_STATE_ESTABLISHED) {
+		/* Discard INIT matching peer vtag after handshake completion (stale INIT). */
+		if (chunk->subh.init_hdr->init_tag == asoc->peer.i.init_tag)
+			return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
+	}
+
 	/* Verify the INIT chunk before processing it. */
 	err_chunk = NULL;
 	if (!sctp_verify_init(net, ep, asoc, chunk->chunk_hdr->type,
-- 
2.47.1