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Message-ID: <CAAVpQUBqiSkPGSGLqDweeOifGUPVx6TvyYcy2BoxKSY2qrtOPg@mail.gmail.com>
Date: Mon, 22 Sep 2025 20:56:11 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: luoxuanqiang <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 v4 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()
On Mon, Sep 22, 2025 at 7:07 PM luoxuanqiang <xuanqiang.luo@...ux.dev> wrote:
>
>
> 在 2025/9/23 08:45, Kuniyuki Iwashima 写道:
> > On Sat, Sep 20, 2025 at 4:00 AM <xuanqiang.luo@...ux.dev> wrote:
> >> From: Xuanqiang Luo <luoxuanqiang@...inos.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@...gle.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@...inos.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.
> + */
> + 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
> >>
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