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

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Message-ID: <20251020060503.325369-3-jiayuan.chen@linux.dev>
Date: Mon, 20 Oct 2025 14:04:47 +0800
From: Jiayuan Chen <jiayuan.chen@...ux.dev>
To: mptcp@...ts.linux.dev,
	netdev@...r.kernel.org,
	bpf@...r.kernel.org
Cc: Jiayuan Chen <jiayuan.chen@...ux.dev>,
	Eric Dumazet <edumazet@...gle.com>,
	Kuniyuki Iwashima <kuniyu@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	John Fastabend <john.fastabend@...il.com>,
	Jakub Sitnicki <jakub@...udflare.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Simon Horman <horms@...nel.org>,
	Matthieu Baerts <matttbe@...nel.org>,
	Mat Martineau <martineau@...nel.org>,
	Geliang Tang <geliang@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	Florian Westphal <fw@...len.de>,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Subject: [PATCH net v2 2/3] bpf,sockmap: disallow MPTCP sockets from sockmap updates

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 <jiayuan.chen@...ux.dev>
---
 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