Re: [PATCH net] tcp: avoid the lookup process failing to get sk in ehash table

[Eric Dumazet <edumazet@google.com>]

On Thu, Jan 12, 2023 at 7:54 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
>
> From: Jason Xing <kernelxing@tencent.com>
>
> While one cpu is working on looking up the right socket from ehash
> table, another cpu is done deleting the request socket and is about
> to add (or is adding) the big socket from the table. It means that
> we could miss both of them, even though it has little chance.
>
> Let me draw a call trace map of the server side.
>    CPU 0                           CPU 1
>    -----                           -----
> tcp_v4_rcv()                  syn_recv_sock()
>                             inet_ehash_insert()
>                             -> sk_nulls_del_node_init_rcu(osk)
> __inet_lookup_established()
>                             -> __sk_nulls_add_node_rcu(sk, list)
>
> Notice that the CPU 0 is receiving the data after the final ack
> during 3-way shakehands and CPU 1 is still handling the final ack.
>
> Why could this be a real problem?
> This case is happening only when the final ack and the first data
> receiving by different CPUs. Then the server receiving data with
> ACK flag tries to search one proper established socket from ehash
> table, but apparently it fails as my map shows above. After that,
> the server fetches a listener socket and then sends a RST because
> it finds a ACK flag in the skb (data), which obeys RST definition
> in RFC 793.
>
> Many thanks to Eric for great help from beginning to end.
>
> Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
>  net/ipv4/inet_hashtables.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index 24a38b56fab9..18f88cb4efcb 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -650,7 +650,16 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
>         spin_lock(lock);
>         if (osk) {
>                 WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
> +               if (sk_hashed(osk))
> +                       /* Before deleting the node, we insert a new one to make
> +                        * sure that the look-up=sk process would not miss either
> +                        * of them and that at least one node would exist in ehash
> +                        * table all the time. Otherwise there's a tiny chance
> +                        * that lookup process could find nothing in ehash table.
> +                        */
> +                       __sk_nulls_add_node_rcu(sk, list);

In our private email exchange, I suggested to insert sk at the _tail_
of the hash bucket.

Inserting it at the _head_ would still leave a race condition, because
a concurrent reader might
have already started the bucket traversal, and would not see 'sk'.

Thanks.

>                 ret = sk_nulls_del_node_init_rcu(osk);
> +               goto unlock;
>         } else if (found_dup_sk) {
>                 *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
>                 if (*found_dup_sk)
> @@ -660,6 +669,7 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
>         if (ret)
>                 __sk_nulls_add_node_rcu(sk, list);
>
> +unlock:
>         spin_unlock(lock);
>
>         return ret;
> --
> 2.37.3
>