netdev - Re: [PATCH v6 bpf-next 3/6] bpf: tcp: Handle BPF SYN Cookie in skb_steal

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Date: Thu, 14 Dec 2023 17:31:15 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v6 bpf-next 3/6] bpf: tcp: Handle BPF SYN Cookie in skb_steal_sock().

On Thu, Dec 14, 2023 at 4:56 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> We will support arbitrary SYN Cookie with BPF.
>
> If BPF prog validates ACK and kfunc allocates a reqsk, it will
> be carried to TCP stack as skb->sk with req->syncookie 1.  Also,
> the reqsk has its listener as req->rsk_listener with no refcnt
> taken.
>
> When the TCP stack looks up a socket from the skb, we steal
> inet_reqsk(skb->sk)->rsk_listener in skb_steal_sock() so that
> the skb will be processed in cookie_v[46]_check() with the
> listener.
>
> Note that we do not clear skb->sk and skb->destructor so that we
> can carry the reqsk to cookie_v[46]_check().
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>  include/net/request_sock.h | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/include/net/request_sock.h b/include/net/request_sock.h
> index 26c630c40abb..8839133d6f6b 100644
> --- a/include/net/request_sock.h
> +++ b/include/net/request_sock.h
> @@ -101,10 +101,21 @@ static inline struct sock *skb_steal_sock(struct sk_buff *skb,
>         }
>
>         *prefetched = skb_sk_is_prefetched(skb);
> -       if (*prefetched)
> +       if (*prefetched) {
> +#if IS_ENABLED(CONFIG_SYN_COOKIES)
> +               if (sk->sk_state == TCP_NEW_SYN_RECV && inet_reqsk(sk)->syncookie) {
> +                       struct request_sock *req = inet_reqsk(sk);
> +
> +                       *refcounted = false;
> +                       sk = req->rsk_listener;
> +                       req->rsk_listener = NULL;

I am not sure about interactions with MPTCP.

I would be nice to have their feedback.

> +                       return sk;
> +               }
> +#endif
>                 *refcounted = sk_is_refcounted(sk);
> -       else
> +       } else {
>                 *refcounted = true;
> +       }
>
>         skb->destructor = NULL;
>         skb->sk = NULL;
> --
> 2.30.2
>