FAILED: patch "[PATCH] mptcp: do not fallback when OoO is present" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 1bba3f219c5e8c29e63afa3c1fc24f875ebec119
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025112425-math-lasso-c3b8@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 1bba3f219c5e8c29e63afa3c1fc24f875ebec119 Mon Sep 17 00:00:00 2001
From: Paolo Abeni <[email protected]>
Date: Tue, 18 Nov 2025 08:20:22 +0100
Subject: [PATCH] mptcp: do not fallback when OoO is present

In case of DSS corruption, the MPTCP protocol tries to avoid the subflow reset if fallback is possible. Such corruptions happen in the receive path; to ensure fallback is possible the stack additionally needs to check for OoO data, otherwise the fallback will break the data stream.

index e30e9043a694..6f0e8f670d83 100644
@@ -76,6 +76,13 @@ bool __mptcp_try_fallback(struct mptcp_sock *msk, int fb_mib)
 	if (__mptcp_check_fallback(msk))
 		return true;
 
+	/* The caller possibly is not holding the msk socket lock, but
+	 * in the fallback case only the current subflow is touching
+	 * the OoO queue.
+	 */
+	if (!RB_EMPTY_ROOT(&msk->out_of_order_queue))
+		return false;
+
 	spin_lock_bh(&msk->fallback_lock);
 	if (!msk->allow_infinite_fallback) {
 		spin_unlock_bh(&msk->fallback_lock);

From: Paolo Abeni <[email protected]>

[ Upstream commit 1bba3f219c5e8c29e63afa3c1fc24f875ebec119 ]

In case of DSS corruption, the MPTCP protocol tries to avoid the subflow reset if fallback is possible. Such corruptions happen in the receive path; to ensure fallback is possible the stack additionally needs to check for OoO data, otherwise the fallback will break the data stream.

---
 net/mptcp/protocol.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 490fd8b188894..54c74c2870b77 100644
@@ -573,6 +573,15 @@ static bool mptcp_check_data_fin(struct sock *sk)
 static void mptcp_dss_corruption(struct mptcp_sock *msk, struct sock *ssk)
 {
 	if (READ_ONCE(msk->allow_infinite_fallback)) {
+		/* The caller possibly is not holding the msk socket lock, but
+		 * in the fallback case only the current subflow is touching
+		 * the OoO queue.
+		 */
+		if (!RB_EMPTY_ROOT(&msk->out_of_order_queue)) {
+			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_DSSCORRUPTIONRESET);
+			mptcp_subflow_reset(ssk);
+			return;
+		}
 		MPTCP_INC_STATS(sock_net(ssk),
 				MPTCP_MIB_DSSCORRUPTIONFALLBACK);
 		mptcp_do_fallback(ssk);

2.51.0