FAILED: patch "[PATCH] mptcp: use MPTCP_RST_EMPTCP for ACK HMAC validation failure" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x a6da02d4c00fdda2417e42ad2b762a9209e6cc49
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026051229-luster-debtor-8bf2@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a6da02d4c00fdda2417e42ad2b762a9209e6cc49 Mon Sep 17 00:00:00 2001
From: Shardul Bankar <[email protected]>
Date: Fri, 1 May 2026 21:35:35 +0200
Subject: [PATCH] mptcp: use MPTCP_RST_EMPTCP for ACK HMAC validation failure

When HMAC validation fails on a received ACK + MP_JOIN in subflow_syn_recv_sock(), the subflow is reset with reason MPTCP_RST_EPROHIBIT ("Administratively prohibited"). This is
incorrect: HMAC validation failure is an MPTCP protocol-level error, not an administrative policy denial.

The mirror site on the client, in subflow_finish_connect(), already uses MPTCP_RST_EMPTCP ("MPTCP-specific error") for the same kind of HMAC failure on the SYN/ACK + MP_JOIN. Use the same reason on the server side for symmetry and accuracy.

index bda6862264ca..d562e149606f 100644
@@ -908,7 +908,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 
 			if (!subflow_hmac_valid(subflow_req, &mp_opt)) {
 				SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINACKMAC);
-				subflow_add_reset_reason(skb, MPTCP_RST_EPROHIBIT);
+				subflow_add_reset_reason(skb, MPTCP_RST_EMPTCP);
 				goto dispose_child;
 			}