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 a3d3fbb78075b848194196fc378b08814ab15ac7..f2e82599dd0a36bbbd5cb67ea1b6b457fc266ea1 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;