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>
---
 net/mptcp/subflow.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 409bd415ef1d190d5599658d01323ad8c8a9be93..24c2de1891bdf31dfe04ef2077113563aad0e666 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -899,13 +899,17 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 				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);
 				subflow_add_reset_reason(skb, MPTCP_RST_EPROHIBIT);
 				goto dispose_child;
 			}
 
+			if (!mptcp_can_accept_new_subflow(owner)) {
+				subflow_add_reset_reason(skb, MPTCP_RST_EPROHIBIT);
+				goto dispose_child;
+			}
+
 			/* move the msk reference ownership to the subflow */
 			subflow_req->msk = NULL;
 			ctx->conn = (struct sock *)owner;

-- 
2.48.1

On Mon, Apr 07, 2025 at 08:26:32PM +0200, Matthieu Baerts (NGI0) wrote:
> 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>