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 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:

@@ -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.
+        */
+       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
>>