Re: [PATCH v6 net] tcp: avoid the lookup process failing to get sk in ehash table
From: Jason Xing
Date: Tue Jan 17 2023 - 20:03:28 EST
On Wed, Jan 18, 2023 at 2:42 AM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
>
> From: Jason Xing <kerneljasonxing@xxxxxxxxx>
> Date: Wed, 18 Jan 2023 01:53:40 +0800
> > From: Jason Xing <kernelxing@xxxxxxxxxxx>
> >
> > 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.
> >
> > Besides, Eric pointed out there's one more race condition where it
> > handles tw socket hashdance. Only by adding to the tail of the list
> > before deleting the old one can we avoid the race if the reader has
> > already begun the bucket traversal and it would possibly miss the head.
> >
> > Many thanks to Eric for great help from beginning to end.
> >
> > Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
> > Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
> > Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> > Link: https://lore.kernel.org/lkml/20230112065336.41034-1-kerneljasonxing@xxxxxxxxx/
> > ---
> > v3,4,5,6:
> > 1) nit: adjust the coding style.
> >
> > v2:
> > 1) add the sk node into the tail of list to prevent the race.
> > 2) fix the race condition when handling time-wait socket hashdance.
> > ---
> > net/ipv4/inet_hashtables.c | 17 +++++++++++++++--
> > net/ipv4/inet_timewait_sock.c | 12 ++++++------
> > 2 files changed, 21 insertions(+), 8 deletions(-)
> >
> > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> > index 24a38b56fab9..f58d73888638 100644
> > --- a/net/ipv4/inet_hashtables.c
> > +++ b/net/ipv4/inet_hashtables.c
> > @@ -650,8 +650,20 @@ 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);
> > - ret = sk_nulls_del_node_init_rcu(osk);
> > - } else if (found_dup_sk) {
> > + ret = sk_hashed(osk);
> > + if (ret) {
> > + /* 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_tail_rcu(sk, list);
> > + sk_nulls_del_node_init_rcu(osk);
> > + }
> > + goto unlock;
> > + }
> > + if (found_dup_sk) {
> > *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
> > if (*found_dup_sk)
> > ret = false;
> > @@ -660,6 +672,7 @@ 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;
> > diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> > index 1d77d992e6e7..b66f2dea5a78 100644
> > --- a/net/ipv4/inet_timewait_sock.c
> > +++ b/net/ipv4/inet_timewait_sock.c
> > @@ -91,20 +91,20 @@ 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,
> > - struct hlist_nulls_head *list)
> > +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,
> > - struct hlist_head *list)
> > + struct hlist_head *list)
> > {
> > hlist_add_head(&tw->tw_bind_node, list);
> > }
> >
> > static void inet_twsk_add_bind2_node(struct inet_timewait_sock *tw,
> > - struct hlist_head *list)
> > + struct hlist_head *list)
> > {
> > hlist_add_head(&tw->tw_bind2_node, list);
> > }
>
> You need not change inet_twsk_add_bind_node() and
I'll drop them and then send a v7 patch.
Thanks,
Jason
> inet_twsk_add_bind2_node().
>
> Thanks,
> Kuniyuki
>
>
> > @@ -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))
> > --
> > 2.37.3