Re: [Patch net] kcm: fix a race condition in kcm_recvmsg()

[Cong Wang <xiyou.wangcong@gmail.com>]

On Tue, Oct 25, 2022 at 04:49:48PM -0700, Eric Dumazet wrote:
> On Tue, Oct 25, 2022 at 4:02 PM Jakub Kicinski <kuba@kernel.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)

rx_lock is acquired at least by 2 callers of it, requeue_rx_msgs() and
kcm_rcv_ready(). kcm_rcv_strparser() seems missing it, I can fix this in
a separate patch as no one actually reported a bug.

Thanks.