[PATCH net 1/4] mptcp: use MPJoinSynAckHMacFailure for SynAck HMAC failure

Matthieu Baerts (NGI0)
2026-05-01T19:36:14Z

From: Shardul Bankar <shardul.b@mpiricsoftware.com>

In subflow_finish_connect(), HMAC validation of the server's HMAC
in SYN/ACK + MP_JOIN increments MPTCP_MIB_JOINACKMAC ("HMAC was
wrong on ACK + MP_JOIN") on failure. The function processes the
SYN/ACK, not the ACK; the matching MPTCP_MIB_JOINSYNACKMAC counter
("HMAC was wrong on SYN/ACK + MP_JOIN") exists but is not
incremented anywhere in the tree.

The mirror site on the server, subflow_syn_recv_sock(), already
uses JOINACKMAC correctly for ACK HMAC failure. Use JOINSYNACKMAC
at the SYN/ACK validation site so each counter reflects the packet
whose HMAC actually failed.

Suggested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Fixes: fc518953bc9c ("mptcp: add and use MIB counter infrastructure")
Cc: stable@vger.kernel.org
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
net/mptcp/subflow.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index e2cb9d23e4a0..bda6862264ca 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -581,7 +581,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
subflow->backup);

if (!subflow_thmac_valid(subflow)) {
- MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINACKMAC);
+ MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINSYNACKMAC);
subflow->reset_reason = MPTCP_RST_EMPTCP;
goto do_reset;
}

--
2.53.0