[PATCH 5.10.y 2/3] mptcp: only inc MPJoinAckHMacFailure for HMAC failures

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

commit 21c02e8272bc95ba0dd44943665c669029b42760 upstream.

Recently, during a debugging session using local MPTCP connections, I
noticed MPJoinAckHMacFailure was not zero on the server side. The
counter was in fact incremented when the PM rejected new subflows,
because the 'subflow' limit was reached.

The fix is easy, simply dissociating the two cases: only the HMAC
validation check should increase MPTCP_MIB_JOINACKMAC counter.

Fixes: 4cf8b7e48a09 ("subflow: introduce and use mptcp_can_accept_new_subflow()")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250407-net-mptcp-hmac-failure-mib-v1-1-3c9ecd0a3a50@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ No conflicts, but subflow_add_reset_reason() is not needed is this
 version: the related feature is not supported in this version. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/subflow.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 5403292d4473..9a8d0d877bd1 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -631,12 +631,14 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 			if (!owner)
 				goto dispose_child;
 
-			if (!subflow_hmac_valid(req, &mp_opt) ||
-			    !mptcp_can_accept_new_subflow(subflow_req->msk)) {
+			if (!subflow_hmac_valid(req, &mp_opt)) {
 				SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINACKMAC);
 				goto dispose_child;
 			}
 
+			if (!mptcp_can_accept_new_subflow(owner))
+				goto dispose_child;
+
 			/* move the msk reference ownership to the subflow */
 			subflow_req->msk = NULL;
 			ctx->conn = (struct sock *)owner;
-- 
2.48.1