Re: [PATCH net-next v4 2/3] inet: Avoid ehash lookup race in inet_ehash_insert()

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

在 2025/9/23 08:23, 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 one CPU performs a lookup while
>> another concurrently deletes and inserts (removing reqsk and inserting sk),
>> the lookup may fail to find the socket, an RST may be sent.
>>
>> The call trace map is drawn as follows:
>>     CPU 0                           CPU 1
>>     -----                           -----
>>                                  inet_ehash_insert()
>>                                  spin_lock()
>>                                  sk_nulls_del_node_init_rcu(osk)
>> __inet_lookup_established()
>>          (lookup failed)
>>                                  __sk_nulls_add_node_rcu(sk, list)
>>                                  spin_unlock()
>>
>> As both deletion and insertion operate on the same ehash chain, this patch
>> introduces two new sk_nulls_replace_* helper functions to implement atomic
>> replacement.
>>
>> Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
>> Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
>> ---
>>   include/net/sock.h         | 23 +++++++++++++++++++++++
>>   net/ipv4/inet_hashtables.c |  4 +++-
>>   2 files changed, 26 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/net/sock.h b/include/net/sock.h
>> index 0fd465935334..e709376eaf0a 100644
>> --- a/include/net/sock.h
>> +++ b/include/net/sock.h
>> @@ -854,6 +854,29 @@ static inline bool sk_nulls_del_node_init_rcu(struct sock *sk)
>>          return rc;
>>   }
>>
>> +static inline bool __sk_nulls_replace_node_init_rcu(struct sock *old,
>> +                                                   struct sock *new)
> nit: This can be inlined into sk_nulls_replace_node_init_rcu() as
> there is no caller of __sk_nulls_replace_node_init_rcu().
>
>
>> +{
>> +       if (sk_hashed(old)) {
>> +               hlist_nulls_replace_init_rcu(&old->sk_nulls_node,
>> +                                            &new->sk_nulls_node);
>> +               return true;
>> +       }
>> +       return false;
>> +}
>> +
>> +static inline bool sk_nulls_replace_node_init_rcu(struct sock *old,
>> +                                                 struct sock *new)
>> +{
>> +       bool rc = __sk_nulls_replace_node_init_rcu(old, new);
>> +
>> +       if (rc) {
>> +               WARN_ON(refcount_read(&old->sk_refcnt) == 1);
> nit: DEBUG_NET_WARN_ON_ONCE() would be better as
> this is "paranoid" as commented in sk_del_node_init() etc.

Fix in next version! Thanks Kuniyuki!

>
>> +               __sock_put(old);
>> +       }
>> +       return rc;
>> +}
>> +
>>   static inline void __sk_add_node(struct sock *sk, struct hlist_head *list)
>>   {
>>          hlist_add_head(&sk->sk_node, list);
>> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
>> index ef4ccfd46ff6..83c9ec625419 100644
>> --- a/net/ipv4/inet_hashtables.c
>> +++ b/net/ipv4/inet_hashtables.c
>> @@ -685,7 +685,8 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
>>          spin_lock(lock);
>>          if (osk) {
>>                  WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
>> -               ret = sk_nulls_del_node_init_rcu(osk);
>> +               ret = sk_nulls_replace_node_init_rcu(osk, sk);
>> +               goto unlock;
>>          } else if (found_dup_sk) {
>>                  *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
>>                  if (*found_dup_sk)
>> @@ -695,6 +696,7 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
>>          if (ret)
>>                  __sk_nulls_add_node_rcu(sk, list);
>>
>> +unlock:
>>          spin_unlock(lock);
>>
>>          return ret;
>> --
>> 2.25.1
>>