Re: [PATCH net-next v1 2/3] inet: Avoid ehash lookup race in inet_ehash_insert()

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

在 2025/9/16 10:18, Kuniyuki Iwashima 写道:
> On Mon, Sep 15, 2025 at 6:57 PM luoxuanqiang <xuanqiang.luo@linux.dev> wrote:
>>
>> 在 2025/9/16 07:00, Kuniyuki Iwashima 写道:
>>> On Mon, Sep 15, 2025 at 12:04 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.
>>>>
>>>> If sk_nulls_replace_node_init_rcu() fails, it indicates osk is either
>>>> hlist_unhashed or hlist_nulls_unhashed. The former returns false; the
>>>> latter performs insertion without deletion.
>>>>
>>>> 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 |  7 +++++++
>>>>    2 files changed, 30 insertions(+)
>>>>
>>>> diff --git a/include/net/sock.h b/include/net/sock.h
>>>> index 896bec2d2176..26dacf7bc93e 100644
>>>> --- a/include/net/sock.h
>>>> +++ b/include/net/sock.h
>>>> @@ -859,6 +859,29 @@ 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)
>>>> +{
>>>> +       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);
>>>> +               __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..7803fd3cc8e9 100644
>>>> --- a/net/ipv4/inet_hashtables.c
>>>> +++ b/net/ipv4/inet_hashtables.c
>>>> @@ -685,6 +685,12 @@ 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);
>>>> +               /* Since osk and sk should be in the same ehash bucket, try
>>>> +                * direct replacement to avoid lookup gaps. On failure, no
>>>> +                * changes. sk_nulls_del_node_init_rcu() will handle the rest.
>>> Both sk_nulls_replace_node_init_rcu() and
>>> sk_nulls_del_node_init_rcu() return true only when
>>> sk_hashed(osk) == true.
>>>
>>> Only thing sk_nulls_del_node_init_rcu() does is to
>>> set ret to false.
>>>
>>>
>>>> +                */
>>>> +               if (sk_nulls_replace_node_init_rcu(osk, sk))
>>>> +                       goto unlock;
>>>>                   ret = sk_nulls_del_node_init_rcu(osk);
>>> So, should we simply do
>>>
>>> ret = sk_nulls_replace_node_init_rcu(osk, sk);
>>> goto unlock;
>>>
>>> ?
>> sk_nulls_replace_node_init_rcu() only returns true if both
>> sk_hashed(osk) == true and hlist_nulls_unhashed(old) == false.
> sk_hashed(sk) == !hlist_nulls_unhashed(&sk->sk_nulls_node)
> is always true as sk_node and sk_nulls_node are in union.
>
>
>> However, in the original sk_nulls_del_node_init_rcu() logic, when
>> sk_hashed(osk) == true,
> So this should be an unreachable branch.

Oh, you're right! That's a point I overlooked. I'll fix it in V2.


thanks Kuni!

>> it always performs __sock_put(sk) regardless of
>> the hlist_nulls_unhashed(old) check. Therefore, if
>> sk_nulls_replace_node_init_rcu() fails, we can safely let ret or
>> __sock_put(sk) be handled by the subsequent
>> sk_nulls_del_node_init_rcu(osk) call. Thanks Xuanqiang.
>>
>>>>           } else if (found_dup_sk) {
>>>>                   *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
>>>> @@ -695,6 +701,7 @@ 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;
>>>> --
>>>> 2.27.0
>>>>