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

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Message-ID: <CAAVpQUBY=h3gDfaX=J9vbSuhYTn8cfCsBGhPLqoer0OSYdihDg@mail.gmail.com>
Date: Tue, 16 Sep 2025 21:36:31 -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 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() ?


> the replacement operation failed, we need
> to insert tw, and this doesn't change the original logic.
>
> >
> >> +
> >>          inet_twsk_schedule(tw, timeo);
> >>
> >>          spin_unlock(lock);
> >> --
> >> 2.25.1
> >>