[PATCH net v3 1/3] net,mptcp: fix proto fallback detection with BPF sockmap

[Jiayuan Chen <jiayuan.chen@linux.dev>]

When the server has MPTCP enabled but receives a non-MP-capable request
from a client, it calls mptcp_fallback_tcp_ops().

Since non-MPTCP connections are allowed to use sockmap, which replaces
sk->sk_prot, using sk->sk_prot to determine the IP version in
mptcp_fallback_tcp_ops() becomes unreliable. This can lead to assigning
incorrect ops to sk->sk_socket->ops.

Additionally, when BPF Sockmap modifies the protocol handlers, the
original WARN_ON_ONCE(sk->sk_prot != &tcp_prot) check would falsely
trigger warnings.

Fix this by using the more stable sk_family to distinguish between IPv4
and IPv6 connections, ensuring correct fallback protocol operations are
selected even when BPF Sockmap has modified the socket protocol handlers.

Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 net/mptcp/protocol.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 0292162a14ee..2393741bc310 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -61,11 +61,16 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk)
 
 static const struct proto_ops *mptcp_fallback_tcp_ops(const struct sock *sk)
 {
+	/* When BPF sockmap is used, it may replace sk->sk_prot.
+	 * Using sk_family is a reliable way to determine the IP version.
+	 */
+	unsigned short family = READ_ONCE(sk->sk_family);
+
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
-	if (sk->sk_prot == &tcpv6_prot)
+	if (family == AF_INET6)
 		return &inet6_stream_ops;
 #endif
-	WARN_ON_ONCE(sk->sk_prot != &tcp_prot);
+	WARN_ON_ONCE(family != AF_INET);
 	return &inet_stream_ops;
 }
 
-- 
2.43.0