Re: [PATCH v6 bpf-next 3/6] bpf: tcp: Handle BPF SYN Cookie in skb_steal_sock().

[Mat Martineau <martineau@kernel.org>]

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On Fri, 15 Dec 2023, Kuniyuki Iwashima wrote:

> From: Eric Dumazet <edumazet@google.com>
> Date: Thu, 14 Dec 2023 17:31:15 +0100
>> On Thu, Dec 14, 2023 at 4:56 PM Kuniyuki Iwashima <kuniyu@amazon.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@amazon.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.
>
> Matthieu, Mat, Paolo, could you double check if the change
> above is sane ?
> https://lore.kernel.org/bpf/20231214155424.67136-4-kuniyu@amazon.com/

Hi Kuniyuki -

Yes, we will take a look. Haven't had time to look in detail yet but I 
wanted to let you know we saw your message and will follow up.

- Mat


>
>
> Short sumamry:
>
> With this series, tc could allocate reqsk to skb->sk and set a
> listener to reqsk->rsk_listener, then __inet_lookup_skb() returns
> a listener in the same reuseport group, and skb is processed in the
> listener function flow, especially cookie_v[46]_check().
>
> The only difference here is that skb->sk has reqsk, which does not
> have rsk_listener.
>
>
>>
>>> +                       return sk;
>>> +               }
>>> +#endif
>>>                 *refcounted = sk_is_refcounted(sk);
>>> -       else
>>> +       } else {
>>>                 *refcounted = true;
>>> +       }
>>>
>>>         skb->destructor = NULL;
>>>         skb->sk = NULL;
>>> --
>>> 2.30.2
>
>