netdev - Re: [PATCH net] ipv6: tcp: drop silly ICMPv6 packet too big messages

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Date:   Wed, 7 Jul 2021 18:25:10 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Willy Tarreau <w@....eu>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Maciej Zenczykowski <maze@...gle.com>
Subject: Re: [PATCH net] ipv6: tcp: drop silly ICMPv6 packet too big messages

On Wed, Jul 7, 2021 at 6:15 PM Willy Tarreau <w@....eu> wrote:
>
> On Wed, Jul 07, 2021 at 06:06:21PM +0200, Eric Dumazet wrote:
> > On Wed, Jul 7, 2021 at 5:59 PM Willy Tarreau <w@....eu> wrote:
> > >
> > > Hi Eric,
> > >
> > > On Wed, Jul 07, 2021 at 08:46:30AM -0700, Eric Dumazet wrote:
> > > > From: Eric Dumazet <edumazet@...gle.com>
> > > >
> > > > While TCP stack scales reasonably well, there is still one part that
> > > > can be used to DDOS it.
> > > >
> > > > IPv6 Packet too big messages have to lookup/insert a new route,
> > > > and if abused by attackers, can easily put hosts under high stress,
> > > > with many cpus contending on a spinlock while one is stuck in fib6_run_gc()
> > >
> > > Just thinking loud, wouldn't it make sense to support randomly dropping
> > > such packets on input (or even better rate-limit them) ? After all, if
> > > a host on the net feels like it will need to send one, it will surely
> > > need to send a few more until one is taken into account so it's not
> > > dramatic. And this could help significantly reduce their processing cost.
> >
> > Not sure what you mean by random.
>
> I just meant statistical randomness. E.g. drop 9/10 when under stress for
> example.

It is hard to define ' stress'. In our case we were maybe receiving 10
ICMPv6 messages per second " only "

I would rather define the issue as a deficiency in current IPv6 stack vs routes.

One can hope that one day the issue will disappear.

>
> > We probably want to process valid packets, if they ever reach us.
>
> That's indeed the other side of my question. I.e. if a server gets hit
> by such a flood, do we consider more important to spend the CPU cycles
> processing all received packets or can we afford dropping a lot of them.
>
> > In our case, we could simply drop all ICMPv6 " packet too big"
> >  messages, since we clamp TCP/IPv6 MSS to the bare minimum anyway.
> >
> > Adding a generic check in TCP/ipv6 stack is cheaper than an iptables
> > rule (especially if this is the only rule that must be used)
>
> Sure, I was not thinking about iptables here, rather a hard-coded
> prandom_u32() call or a percpu cycling counter.
>
> Willy