netdev - Re: [Patch net] kcm: fix a race condition in kcm

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Date:   Tue, 25 Oct 2022 16:49:48 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org,
        Cong Wang <cong.wang@...edance.com>,
        shaozhengchao <shaozhengchao@...wei.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [Patch net] kcm: fix a race condition in kcm_recvmsg()

On Tue, Oct 25, 2022 at 4:02 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sat, 22 Oct 2022 19:30:44 -0700 Cong Wang wrote:
> > +                     spin_lock_bh(&mux->rx_lock);
> >                       KCM_STATS_INCR(kcm->stats.rx_msgs);
> >                       skb_unlink(skb, &sk->sk_receive_queue);
> > +                     spin_unlock_bh(&mux->rx_lock);
>
> Why not switch to __skb_unlink() at the same time?
> Abundance of caution?
>
> Adding Eric who was fixing KCM bugs recently.

I think kcm_queue_rcv_skb() might have a similar problem if/when
called from requeue_rx_msgs()

(The mux->rx_lock spinlock is not acquired, and skb_queue_tail() is used)

I agree we should stick to one lock, and if this is not the standard
skb head lock, we should not use it at all
(ie use __skb_queue_tail() and friends)