Re: [PATCH net-next v4 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

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

在 2025/9/23 11:56, Kuniyuki Iwashima 写道:
> On Mon, Sep 22, 2025 at 7:07 PM luoxuanqiang <xuanqiang.luo@linux.dev> wrote:
>>
>> 在 2025/9/23 08:45, Kuniyuki Iwashima 写道:
>>> On Sat, Sep 20, 2025 at 4:00 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.
>>>>
>>>> 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@google.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.
>> Got it, but still really appreciate your detailed
>> and patient review!
>>
>>>> Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.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.
>>>
>> Thanks for Eric's pointer!
>>
>> Could you let me know if my modification here works?
>>
>> That is, moving smp_wmb() to after the refcount update:
> I think this should be fine, small comment below
>
>> @@ -140,19 +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().
>> -        */
>> -       smp_wmb();
>>           /* tw_refcnt is set to 3 because we have :
>>            * - one reference for bhash chain.
>>            * - one reference for ehash chain.
>> @@ -162,6 +149,14 @@ 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 by
>> +        * necessary memory barrier.
> This sounds like tw is published by memory barrier,
> perhaps remove after 'by' ?  It's obvious that the comment
> is for smp_wmb() below.

I'm sorry for the confusion caused by my poor English.

I will express it more clearly in the next version,
like the following:

@@ -162,6 +149,15 @@ 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);

Thanks!
Xuanqiang

>
>
>> +        */
>> +       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);
>>
>> Thanks!
>> Xuanqiang
>>
>>>> +       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
>>>>