Re: [RESEND PATCH net-next v7 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

October 14, 2025 at 10:27, xuanqiang.luo@linux.dev mailto: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")
> Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
> Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>