Re: [PATCH net-next v6 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

[luoxuanqiang <xuanqiang.luo@linux.dev>]

在 2025/9/26 02:22, Kuniyuki Iwashima 写道:
> On Wed, Sep 24, 2025 at 7:18 PM <xuanqiang.luo@linux.dev> wrote:
>> From: Xuanqiang Luo <luoxuanqiang@kylinos.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@kylinos.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@google.com>
>
I'll fix the comment style in the next version.

Thanks for the review!

>
>
>>           */
>> @@ -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
>>