Re: [PATCH net-next v3 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

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

在 2025/9/17 03:48, Kuniyuki Iwashima 写道:
> On Tue, Sep 16, 2025 at 3:31 AM <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.
>>
>> 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 | 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, 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
>>