netdev - Re: [PATCH net-next v6 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance

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Message-ID: <CAAVpQUD7-6hgCSvhP3KL+thgxcyWAJQanfPHS+BQ5LDfrY9-bQ@mail.gmail.com>
Date: Thu, 25 Sep 2025 11:22:42 -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 v6 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

On Wed, Sep 24, 2025 at 7:18 PM <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.
>
> It's worth noting that we held lock_sock() before the replacement, so
> there's no need to check if sk is hashed. Thanks to Kuniyuki Iwashima!
>
> 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 | 31 ++++++++++---------------------
>  1 file changed, 10 insertions(+), 21 deletions(-)
>
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index 5b5426b8ee92..89dc0a5d7248 100644
> --- a/net/ipv4/inet_timewait_sock.c
> +++ b/net/ipv4/inet_timewait_sock.c
> @@ -87,12 +87,6 @@ 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)
> -{
> -       hlist_nulls_add_head_rcu(&tw->tw_node, list);
> -}
> -
>  static void inet_twsk_schedule(struct inet_timewait_sock *tw, int timeo)
>  {
>         __inet_twsk_schedule(tw, timeo, false);
> @@ -112,11 +106,10 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
>  {
>         const struct inet_sock *inet = inet_sk(sk);
>         const struct inet_connection_sock *icsk = inet_csk(sk);
> -       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;
>
> -       /* 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.

While at it, could you update the comment style at these 2 line above too ?

/* Put ..
 * Note, ...
 * binding ...
 */

Otherwise looks good, thanks.

Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>




>          */
> @@ -140,19 +133,6 @@ 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().
> -        */
> -       smp_wmb();
>         /* tw_refcnt is set to 3 because we have :
>          * - one reference for bhash chain.
>          * - one reference for ehash chain.
> @@ -162,6 +142,15 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
>          */
>         refcount_set(&tw->tw_refcnt, 3);
>
> +       /* Ensure tw_refcnt has been set before tw is published.
> +        * smp_wmb() provides the necessary memory barrier to enforce this
> +        * ordering.
> +        */
> +       smp_wmb();
> +
> +       hlist_nulls_replace_init_rcu(&sk->sk_nulls_node, &tw->tw_node);
> +       sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
> +
>         inet_twsk_schedule(tw, timeo);
>
>         spin_unlock(lock);
> --
> 2.25.1
>