netdev - Re: [PATCH net-next v3 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance

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Message-ID: <CAAVpQUCAfZNd94eXuZs1FuQJq-3-Li0td1gg5Eiave+Sv7wBBg@mail.gmail.com>
Date: Fri, 19 Sep 2025 01:38:47 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: luoxuanqiang <xuanqiang.luo@...ux.dev>
Cc: edumazet@...gle.com, kerneljasonxing@...il.com, davem@...emloft.net, 
	kuba@...nel.org, netdev@...r.kernel.org, 
	Xuanqiang Luo <luoxuanqiang@...inos.cn>
Subject: Re: [PATCH net-next v3 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

On Thu, Sep 18, 2025 at 1:33 AM luoxuanqiang <xuanqiang.luo@...ux.dev> wrote:
>
>
> 在 2025/9/17 12:36, Kuniyuki Iwashima 写道:
> > On Tue, Sep 16, 2025 at 8:27 PM luoxuanqiang <xuanqiang.luo@...ux.dev> wrote:
> >>
> >> 在 2025/9/17 03:48, Kuniyuki Iwashima 写道:
> >>> On Tue, Sep 16, 2025 at 3:31 AM <xuanqiang.luo@...ux.dev> wrote:
> >>>> From: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> >>>>
> >>>> Since ehash lookups are lockless, if another CPU is converting sk to tw
> >>>> concurrently, fetching the newly inserted tw with tw->tw_refcnt == 0 cause
> >>>> lookup failure.
> >>>>
> >>>> The call trace map is drawn as follows:
> >>>>      CPU 0                                CPU 1
> >>>>      -----                                -----
> >>>>                                        inet_twsk_hashdance_schedule()
> >>>>                                        spin_lock()
> >>>>                                        inet_twsk_add_node_rcu(tw, ...)
> >>>> __inet_lookup_established()
> >>>> (find tw, failure due to tw_refcnt = 0)
> >>>>                                        __sk_nulls_del_node_init_rcu(sk)
> >>>>                                        refcount_set(&tw->tw_refcnt, 3)
> >>>>                                        spin_unlock()
> >>>>
> >>>> By replacing sk with tw atomically via hlist_nulls_replace_init_rcu() after
> >>>> setting tw_refcnt, we ensure that tw is either fully initialized or not
> >>>> visible to other CPUs, eliminating the race.
> >>>>
> >>>> Fixes: 3ab5aee7fe84 ("net: Convert TCP & DCCP hash tables to use RCU / hlist_nulls")
> >>>> Signed-off-by: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> >>>> ---
> >>>>    net/ipv4/inet_timewait_sock.c | 15 ++++++---------
> >>>>    1 file changed, 6 insertions(+), 9 deletions(-)
> >>>>
> >>>> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> >>>> index 5b5426b8ee92..1ba20c4cb73b 100644
> >>>> --- a/net/ipv4/inet_timewait_sock.c
> >>>> +++ b/net/ipv4/inet_timewait_sock.c
> >>>> @@ -116,7 +116,7 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
> >>>>           spinlock_t *lock = inet_ehash_lockp(hashinfo, sk->sk_hash);
> >>>>           struct inet_bind_hashbucket *bhead, *bhead2;
> >>>>
> >>>> -       /* Step 1: Put TW into bind hash. Original socket stays there too.
> >>>> +       /* Put TW into bind hash. Original socket stays there too.
> >>>>              Note, that any socket with inet->num != 0 MUST be bound in
> >>>>              binding cache, even if it is closed.
> >>>>            */
> >>>> @@ -140,14 +140,6 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
> >>>>
> >>>>           spin_lock(lock);
> >>>>
> >>>> -       /* Step 2: Hash TW into tcp ehash chain */
> >>>> -       inet_twsk_add_node_rcu(tw, &ehead->chain);
> >>>> -
> >>>> -       /* Step 3: Remove SK from hash chain */
> >>>> -       if (__sk_nulls_del_node_init_rcu(sk))
> >>>> -               sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
> >>>> -
> >>>> -
> >>>>           /* Ensure above writes are committed into memory before updating the
> >>>>            * refcount.
> >>>>            * Provides ordering vs later refcount_inc().
> >>>> @@ -162,6 +154,11 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
> >>>>            */
> >>>>           refcount_set(&tw->tw_refcnt, 3);
> >>>>
> >>>> +       if (hlist_nulls_replace_init_rcu(&sk->sk_nulls_node, &tw->tw_node))
> >>>> +               sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
> >>>> +       else
> >>>> +               inet_twsk_add_node_rcu(tw, &ehead->chain);
> >>> When hlist_nulls_replace_init_rcu() returns false ?
> >> When hlist_nulls_replace_init_rcu() returns false, it means
> >> sk is unhashed,
> > and how does this happen ?
> >
> > Here is under lock_sock() I think, for example, you can
> > find a lockdep annotation in the path:
> >
> > tcp_time_wait_init
> >    tp->af_specific->md5_lookup / tcp_v4_md5_lookup
> >      tcp_md5_do_lookup
> >        __tcp_md5_do_lookup
> >          rcu_dereference_check(tp->md5sig_info, lockdep_sock_is_held(sk));
> >
> > So, is there a path that unhashes socket without holding
> > lock_sock() ?
> >
> I'm not entirely sure about this point yet, because
> inet_unhash() is called in too many places and uses
> __sk_nulls_del_node_init_rcu() to unhash sockets without
> explicitly requiring bh_lock_sock().
>
> Until I can verify this, I'll keep the original check
> for old socket unhashed state to ensure safety.
>
> It would be great if you could confirm this behavior.

See:
https://lore.kernel.org/netdev/20250919083706.1863217-4-kuniyu@google.com/