Subject: [PATCH net v2 2/3] bpf,sockmap: disallow MPTCP sockets from sockmap updates

From: Jiayuan Chen <[email-masked]>
Date: Mon, 20 Oct 2025 14:04:47 +0800

MPTCP creates subflows for data transmission, and these sockets should not
be added to sockmap because MPTCP sets specialized data_ready handlers
that would be overridden by sockmap.

Additionally, for the parent socket of MPTCP subflows (plain TCP socket),
MPTCP sk requires specific protocol handling that conflicts with sockmap's
operation(mptcp_prot).

This patch adds proper checks to reject MPTCP subflows and their parent
sockets from being added to sockmap, while preserving compatibility with
reuseport functionality for listening MPTCP sockets.

Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
Signed-off-by: Jiayuan Chen <[email-masked]>
---
 net/core/sock_map.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index 5947b38e4f8b..da21deb970b3 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -535,6 +535,15 @@ static bool sock_map_redirect_allowed(const struct sock *sk)
 
 static bool sock_map_sk_is_suitable(const struct sock *sk)
 {
+	if ((sk_is_tcp(sk) && sk_is_mptcp(sk)) /* subflow */ ||
+	    (sk->sk_protocol == IPPROTO_MPTCP && sk->sk_state != TCP_LISTEN)) {
+		/* Disallow MPTCP subflows and their parent socket.
+		 * However, a TCP_LISTEN MPTCP socket is permitted because
+		 * sockmap can also serve for reuseport socket selection.
+		 */
+		pr_err_once("sockmap: MPTCP sockets are not supported\n");
+		return false;
+	}
 	return !!sk->sk_prot->psock_update_sk_prot;
 }
 
-- 
2.43.0