netdev - Re: [PATCH net-next v6 2/3] inet: Avoid ehash lookup race in inet_ehash

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Message-ID: <CAAVpQUBKLzVWs_gNZ-KUn9zjkyck5NBGQWsD+Am7kK7s3LZTWA@mail.gmail.com>
Date: Thu, 25 Sep 2025 11:00:37 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: xuanqiang.luo@...ux.dev
Cc: edumazet@...gle.com, kerneljasonxing@...il.com, davem@...emloft.net, 
	kuba@...nel.org, netdev@...r.kernel.org, 
	Xuanqiang Luo <luoxuanqiang@...inos.cn>
Subject: Re: [PATCH net-next v6 2/3] inet: Avoid ehash lookup race in inet_ehash_insert()

On Wed, Sep 24, 2025 at 7:18 PM <xuanqiang.luo@...ux.dev> wrote:
>
> From: Xuanqiang Luo <luoxuanqiang@...inos.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 a new sk_nulls_replace_node_init_rcu() helper functions to
> implement atomic replacement.
>
> Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
> Signed-off-by: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> ---
>  include/net/sock.h         | 14 ++++++++++++++
>  net/ipv4/inet_hashtables.c |  4 +++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 0fd465935334..5d67f5cbae52 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -854,6 +854,20 @@ 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);
> +               DEBUG_NET_WARN_ON_ONCE(refcount_read(&old->sk_refcnt) == 1);
> +               __sock_put(old);
> +               return true;
> +       }
> +
> +       return false;
> +}
> +
>  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) {

As you need another iteration, remove else here since
we bail out above with goto:

    goto unlock;
}

if (found_dup_sk) {

Other than that

Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>

Thanks!


>                 *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
>