Re: [PATCH net v2 2/3] bpf,sockmap: disallow MPTCP sockets from sockmap updates

[Jakub Sitnicki <jakub@cloudflare.com>]

On Mon, Oct 20, 2025 at 02:04 PM +08, Jiayuan Chen wrote:
> MPTCP creates subflows for data transmission, and these sockets should not
> be added to sockmap because MPTCP sets specialized data_ready handlers
> that would be overridden by sockmap.
>
> Additionally, for the parent socket of MPTCP subflows (plain TCP socket),
> MPTCP sk requires specific protocol handling that conflicts with sockmap's
> operation(mptcp_prot).
>
> This patch adds proper checks to reject MPTCP subflows and their parent
> sockets from being added to sockmap, while preserving compatibility with
> reuseport functionality for listening MPTCP sockets.
>
> Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  net/core/sock_map.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 5947b38e4f8b..da21deb970b3 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -535,6 +535,15 @@ static bool sock_map_redirect_allowed(const struct sock *sk)
>  
>  static bool sock_map_sk_is_suitable(const struct sock *sk)
>  {
> +	if ((sk_is_tcp(sk) && sk_is_mptcp(sk)) /* subflow */ ||
> +	    (sk->sk_protocol == IPPROTO_MPTCP && sk->sk_state != TCP_LISTEN)) {
> +		/* Disallow MPTCP subflows and their parent socket.
> +		 * However, a TCP_LISTEN MPTCP socket is permitted because
> +		 * sockmap can also serve for reuseport socket selection.
> +		 */
> +		pr_err_once("sockmap: MPTCP sockets are not supported\n");
> +		return false;
> +	}
>  	return !!sk->sk_prot->psock_update_sk_prot;
>  }

You're checking sk_state without sk_lock held. That doesn't seem right.
Take a look how we always call sock_map_sk_state_allowed() after
grabbing the lock.

Same might apply to sk_is_mptcp(). Please double check.