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

[Matthieu Baerts <matttbe@kernel.org>]

Hi Jiayuan,

On 23/10/2025 14:54, 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")
> 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;

Just to be sure: is there anything in BPF modifying sk->sk_socket->ops?
Because that's what mptcp_fallback_tcp_ops() will do somehow.

In other words, is it always fine to set inet(6)_stream_ops? (I guess
yes, but better to be sure while we are looking at that :) )

>  }
>  

Cheers,
Matt
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