netdev - Re: [PATCH net-next v4 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance

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Message-ID: <CAAVpQUDaYX5ZQN+EYL3q4yeu0Ni2cqNODEY--Wb-2+yY650Mbw@mail.gmail.com>
Date: Mon, 22 Sep 2025 17:45:31 -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 v4 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

On Sat, Sep 20, 2025 at 4:00 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.
>
> It's worth noting that we replace under lock_sock(), so 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")
> Suggested-by: Kuniyuki Iwashima <kuniyu@...gle.com>

This is not needed.  A pure review does not deserve Suggested-by.
This is used when someone suggests changing the core idea of
the patch.


> Signed-off-by: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> ---
>  net/ipv4/inet_timewait_sock.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index 5b5426b8ee92..bb98888584a8 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 +154,9 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
>          */
>         refcount_set(&tw->tw_refcnt, 3);

I discussed this series with Eric last week, and he pointed out
(thanks!) that we need to be careful here about memory barrier.

refcount_set() is just WRITE_ONCE() and thus can be reordered,
and twsk could be published with 0 refcnt, resulting in another RST.


>
> +       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
>