On Mon, Oct 20, 2025 at 02:04 PM +08, Jiayuan Chen wrote:
> 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")
> Signed-off-by: Jiayuan Chen <[email protected]>
> ---
> net/mptcp/protocol.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index 0292162a14ee..c2d1513615ae 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -61,11 +61,14 @@ 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 replaces sk->sk_prot.
> + * Using sk_family is a reliable way to determine the IP version.
> + */
> #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_ONCE(sk->sk_family != AF_INET);
> return &inet_stream_ops;
> }
Should probably be a READ_ONCE(sk->sk_family) based on what I see in
IPV6_ADDRFORM:
https://elixir.bootlin.com/linux/v6.18-rc1/source/net/ipv6/ipv6_sockglue.c#L607
Nit: It's BPF sockmap, cpumap, etc. We don't treat it as a proper noun.
Other than that:
Reviewed-by: Jakub Sitnicki <[email protected]>
