Re: [PATCH net] tcp: avoid creating multiple req socks with the same tuples

[Eric Dumazet <edumazet@google.com>]

On Tue, Jun 4, 2019 at 7:47 AM Mao Wenan <maowenan@huawei.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@huawei.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.

I am afraid your patch does not solve all races.

The lookup you add is lockless, so this is racy.

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())

This needs more tricky changes than your patch.