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

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Message-ID: <c7a91e0c-b575-43b3-af5b-64fc35b46fac@linux.dev>
Date: Thu, 18 Sep 2025 16:32:32 +0800
From: luoxuanqiang <xuanqiang.luo@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
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()


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.

Thanks
Xuanqiang.

>> 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
>>>>