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

[Jason Xing <kerneljasonxing@gmail.com>]

On Sat, Jan 14, 2023 at 8:31 PM Eric Dumazet <edumazet@google.com> wrote:
>
> ()
>
> On Sat, Jan 14, 2023 at 1:06 PM Jason Xing <kerneljasonxing@gmail.com> wrote:
> >
> > On Sat, Jan 14, 2023 at 5:45 PM Eric Dumazet <edumazet@google.com> wrote:
> > >
> > > 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.
> > >
> >
> > Yes, I noticed that. At that time I kept considering the race
> > condition of the RCU itself, not the scene you mentioned as below.
> >
> > > 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 for the detailed explanation. Now I see why. I'll replace it
> > with __sk_nulls_add_node_tail_rcu() function and send the v2 patch.
> >
> > By the way, I checked the removal of TIMEWAIT socket which is included
> > in this patch.
> > I write down the call-trace:
> > inet_hash_connect()
> >     -> __inet_hash_connect()
> >         -> if (sk_unhashed(sk)) {
> >                 inet_ehash_nolisten(sk, (struct sock *)tw, NULL);
> >                     -> inet_ehash_insert(sk, osk, found_dup_sk);
> > Therefore, this patch covers the timewait case.
>
> This is the path handling the TIME_WAIT ---> ESTABLISH case.
>
> I was referring to the more common opposite case which is the case
> where a race could possibly happen.
>
> This is inet_twsk_hashdance, and I suspect we want something like:
>

Thanks, Eric. I learned :)

> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index 1d77d992e6e77f7d96bd061be6dbb802c2566b3f..6d681ef52bb24b984a9dbda25b19291fc4393914
> 100644
> --- a/net/ipv4/inet_timewait_sock.c
> +++ b/net/ipv4/inet_timewait_sock.c
> @@ -91,10 +91,10 @@ void inet_twsk_put(struct inet_timewait_sock *tw)
>  }
>  EXPORT_SYMBOL_GPL(inet_twsk_put);
>
> -static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
> +static void inet_twsk_add_node_tail_rcu(struct inet_timewait_sock *tw,
>                                    struct hlist_nulls_head *list)
>  {
> -       hlist_nulls_add_head_rcu(&tw->tw_node, list);
> +       hlist_nulls_add_tail_rcu(&tw->tw_node, list);
>  }
>
>  static void inet_twsk_add_bind_node(struct inet_timewait_sock *tw,
> @@ -147,7 +147,7 @@ void inet_twsk_hashdance(struct inet_timewait_sock
> *tw, struct sock *sk,
>
>         spin_lock(lock);
>
> -       inet_twsk_add_node_rcu(tw, &ehead->chain);
> +       inet_twsk_add_node_tail_rcu(tw, &ehead->chain);
>
>         /* Step 3: Remove SK from hash chain */
>         if (__sk_nulls_del_node_init_rcu(sk))

I'll put this part of the code into my next submission and add more
comments about it.

Thanks,
Jason