netdev - Re: PROBLEM: DoS Attack on Fragment Cache

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Message-ID: <CANn89iKXYutm20oi-rCwch0eL1Oo9rq1W=ex6+NzvPitq_jX0Q@mail.gmail.com>
Date:   Mon, 19 Apr 2021 11:43:15 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Matt Corallo <netdev-list@...tcorallo.com>
Cc:     Willy Tarreau <w@....eu>, Keyu Man <kman001@....edu>,
        David Ahern <dsahern@...il.com>,
        Florian Westphal <fw@...len.de>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Zhiyun Qian <zhiyunq@...ucr.edu>
Subject: Re: PROBLEM: DoS Attack on Fragment Cache

On Sun, Apr 18, 2021 at 4:31 PM Matt Corallo
<netdev-list@...tcorallo.com> wrote:
>
> Should the default, though, be so low? If someone is still using a old modem they can crank up the sysctl, it does seem
> like such things are pretty rare these days :). Its rather trivial to, without any kind of attack, hit 1Mbps of lost
> fragments in today's networks, at which point all fragments are dropped. After all, I submitted the patch to "scratch my
> own itch" :).

Again, even if you increase the values by 1000x, it is trivial for an
attacker to use all the memory you allowed.

And allowing a significant portion of memory to be eaten like that
might cause OOM on hosts where jobs are consuming all physical memory.

It is a sysctl, I changed things so that one could really reserve/use
16GB of memory if she/he is desperate about frags.

>
> Matt
>
> On 4/18/21 00:39, Willy Tarreau wrote:
> > I do agree that we shouldn't keep them that long nowadays, we can't go
> > too low without risking to break some slow transmission stacks (SLIP/PPP
> > over modems for example).