Re: possible race in __inet_lookup_established()

[Michal Kubecek <mkubecek@suse.cz>]

On Wed, Nov 20, 2019 at 12:57:48PM -0800, Eric Dumazet wrote:
> On Wed, Nov 20, 2019 at 12:49 PM Michal Kubecek <mkubecek@suse.cz> wrote:
> > Firo suggested something like
> >
> > ------------------------------------------------------------------------
> > --- a/net/ipv4/inet_hashtables.c
> > +++ b/net/ipv4/inet_hashtables.c
> > @@ -362,6 +362,8 @@ struct sock *__inet_lookup_established(struct net *net,
> >
> >  begin:
> >         sk_nulls_for_each_rcu(sk, node, &head->chain) {
> > +               if (unlikely(!node))
> > +                       goto begin;
> >                 if (sk->sk_hash != hash)
> >                         continue;
> >                 if (likely(INET_MATCH(sk, net, acookie,
> > ------------------------------------------------------------------------
> >
> > It depends on implementation details but I believe it would work. It
> > would be nicer if we could detect the switch to a listening socket but
> > I don't see how to make such test race free without introducing
> > unacceptable performance penalty.
> 
> No, we do not want to add more checks in the fast path really.
> 
> I was more thinking about not breaking the RCU invariants.
> 
> (ie : adding back the nulls stuff that I removed in 3b24d854cb35
> ("tcp/dccp: do not touch
> listener sk_refcnt under synflood")

Yes, that would do the trick. It would add some cycles to listener
lookup but that is less harm than slowing down established socket
lookup.

Michal