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; }