mptcp: only inc MPJoinAckHMacFailure for HMAC failures

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 4011eb6aecd064a44ef8ab1147efeb4edf9f57b9..4c2aa45c466d93b25c4f76ac418c850190d20559 100644 (file)
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -902,13 +902,17 @@ create_child:
                                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;