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Date:   Tue, 4 Jun 2019 20:16:00 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     maowenan <maowenan@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] tcp: avoid creating multiple req socks with the same tuples

On Tue, Jun 4, 2019 at 7:07 PM maowenan <maowenan@...wei.com> wrote:
>
>
>
> On 2019/6/4 23:24, Eric Dumazet wrote:
> > On Tue, Jun 4, 2019 at 7:47 AM Mao Wenan <maowenan@...wei.com> wrote:
> >>
> >> There is one issue about bonding mode BOND_MODE_BROADCAST, and
> >> two slaves with diffierent affinity, so packets will be handled
> >> by different cpu. These are two pre-conditions in this case.
> >>
> >> When two slaves receive the same syn packets at the same time,
> >> two request sock(reqsk) will be created if below situation happens:
> >> 1. syn1 arrived tcp_conn_request, create reqsk1 and have not yet called
> >> inet_csk_reqsk_queue_hash_add.
> >> 2. syn2 arrived tcp_v4_rcv, it goes to tcp_conn_request and create reqsk2
> >> because it can't find reqsk1 in the __inet_lookup_skb.
> >>
> >> Then reqsk1 and reqsk2 are added to establish hash table, and two synack with different
> >> seq(seq1 and seq2) are sent to client, then tcp ack arrived and will be
> >> processed in tcp_v4_rcv and tcp_check_req, if __inet_lookup_skb find the reqsk2, and
> >> tcp ack packet is ack_seq is seq1, it will be failed after checking:
> >> TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1)
> >> and then tcp rst will be sent to client and close the connection.
> >>
> >> To fix this, do lookup before calling inet_csk_reqsk_queue_hash_add
> >> to add reqsk2 to hash table, if it finds the existed reqsk1 with the same five tuples,
> >> it removes reqsk2 and does not send synack to client.
> >>
> >> Signed-off-by: Mao Wenan <maowenan@...wei.com>
> >> ---
> >>  net/ipv4/tcp_input.c | 9 +++++++++
> >>  1 file changed, 9 insertions(+)
> >>
> >> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> >> index 08a477e74cf3..c75eeb1fe098 100644
> >> --- a/net/ipv4/tcp_input.c
> >> +++ b/net/ipv4/tcp_input.c
> >> @@ -6569,6 +6569,15 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
> >>                 bh_unlock_sock(fastopen_sk);
> >>                 sock_put(fastopen_sk);
> >>         } else {
> >> +               struct sock *sk1 = req_to_sk(req);
> >> +               struct sock *sk2 = NULL;
> >> +               sk2 = __inet_lookup_established(sock_net(sk1), &tcp_hashinfo,
> >> +                                                                       sk1->sk_daddr, sk1->sk_dport,
> >> +                                                                       sk1->sk_rcv_saddr, sk1->sk_num,
> >> +                                                                       inet_iif(skb),inet_sdif(skb));
> >> +               if (sk2 != NULL)
> >> +                       goto drop_and_release;
> >> +
> >>                 tcp_rsk(req)->tfo_listener = false;
> >>                 if (!want_cookie)
> >>                         inet_csk_reqsk_queue_hash_add(sk, req,
> >
> > This issue has been discussed last year.
> Can you share discussion information?


https://www.spinics.net/lists/netdev/msg507423.html


>
> >
> > I am afraid your patch does not solve all races.
> >
> > The lookup you add is lockless, so this is racy.
> it's right, it has already in race region.
> >
> > Really the only way to solve this is to make sure that _when_ the
> > bucket lock is held,
> > we do not insert a request socket if the 4-tuple is already in the
> > chain (probably in inet_ehash_insert())
> >
>
> put lookup code in spin_lock() of inet_ehash_insert(), is it ok like this?
> will it affect performance?
>
> in inet_ehash_insert():
> ...
>         spin_lock(lock);
> +       reqsk = __inet_lookup_established(sock_net(sk), &tcp_hashinfo,
> +                                                       sk->sk_daddr, sk->sk_dport,
> +                                                       sk->sk_rcv_saddr, sk->sk_num,
> +                                                       sk_bound_dev_if, sk_bound_dev_if);
> +       if (reqsk) {

You should test this before asking :)


> +               spin_unlock(lock);
> +               return ret;
> +       }
> +
>         if (osk) {
>                 WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
>                 ret = sk_nulls_del_node_init_rcu(osk);
>         }
>         if (ret)
>                 __sk_nulls_add_node_rcu(sk, list);
>         spin_unlock(lock);
> ...
>
> > This needs more tricky changes than your patch.
> >
> > .
> >
>

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