netdev - Re: [PATCH net-next v1 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance

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Message-ID: <CAAVpQUDJU7arjM_7LjrLY-5tNZ0Ch_ZPXCvmmaVD+rBb3LvxHw@mail.gmail.com>
Date: Mon, 15 Sep 2025 16:25:45 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: 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 v1 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

On Mon, Sep 15, 2025 at 12:04 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 | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index 5b5426b8ee92..4c84a020315d 100644
> --- a/net/ipv4/inet_timewait_sock.c
> +++ b/net/ipv4/inet_timewait_sock.c
> @@ -115,8 +115,9 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
>         struct inet_ehash_bucket *ehead = inet_ehash_bucket(hashinfo, sk->sk_hash);
>         spinlock_t *lock = inet_ehash_lockp(hashinfo, sk->sk_hash);
>         struct inet_bind_hashbucket *bhead, *bhead2;
> +       bool ret = false;
>
> -       /* 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 +141,6 @@ 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 +155,15 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
>          */
>         refcount_set(&tw->tw_refcnt, 3);
>
> +       if (sk_hashed(sk)) {

Is this check necessary ?

I think tcp_time_wait() is called under bh_lock_sock() and there
will be no racing thread unlike reqsk vs full sk for 3WHS.

I guess the existing code checks the retval of
__sk_nulls_del_node_init_rcu() just in case.


> +               ret = hlist_nulls_replace_init_rcu(&sk->sk_nulls_node,
> +                                                  &tw->tw_node);
> +               sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
> +       }
> +
> +       if (!ret)
> +               inet_twsk_add_node_rcu(tw, &ehead->chain);
> +
>         inet_twsk_schedule(tw, timeo);
>
>         spin_unlock(lock);
> --
> 2.27.0
>