Subject: [PATCH net-next v1] mptcp: fix incorrect IPv4/IPv6 check

From: Jiayuan Chen <[email-masked]>
Date: Tue, 14 Oct 2025 20:26:18 +0800

When MPTCP falls back to normal TCP, it needs to reset proto_ops. However,
for sockmap and TLS, they have their own custom proto_ops, so simply
checking sk->sk_prot is insufficient.

For example, an IPv6 request might incorrectly follow the IPv4 code path,
leading to kernel panic.

Note that Golang has enabled MPTCP by default [1]

[1] https://go-review.googlesource.com/c/go/+/607715

Fixes: 8e2b8a9fa512 ("mptcp: don't overwrite sock_ops in mptcp_is_tcpsk()")
Signed-off-by: Jiayuan Chen <[email-masked]>
---
 net/mptcp/protocol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 0292162a14ee..efcdaeff91f8 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -62,10 +62,10 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk)
 static const struct proto_ops *mptcp_fallback_tcp_ops(const struct sock *sk)
 {
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
-	if (sk->sk_prot == &tcpv6_prot)
+	if (sk->sk_family == AF_INET6)
 		return &inet6_stream_ops;
 #endif
-	WARN_ON_ONCE(sk->sk_prot != &tcp_prot);
+	WARN_ON(sk->sk_family != AF_INET);
 	return &inet_stream_ops;
 }
 
-- 
2.43.0