netdev - Re: [Patch nf 3/3] xt_hashlimit: limit the max size of hashtable

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Date:   Sun, 2 Feb 2020 10:27:34 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        NetFilter <netfilter-devel@...r.kernel.org>,
        syzbot <syzbot+adf6c6c2be1c3a718121@...kaller.appspotmail.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>
Subject: Re: [Patch nf 3/3] xt_hashlimit: limit the max size of hashtable

On Sat, Feb 1, 2020 at 10:16 PM Florian Westphal <fw@...len.de> wrote:
>
> Cong Wang <xiyou.wangcong@...il.com> wrote:
> > > In order to prevent breaking userspace, perhaps make it so that the
> > > kernel caps cfg.max at twice that value?  Would allow storing up to
> > > 16777216 addresses with an average chain depth of 16 (which is quite
> > > large).  We could increase the max limit in case someone presents a use
> > > case.
> > >
> >
> > Not sure if I understand this, I don't see how cap'ing cfg->max could
> > help prevent breaking user-space? Are you suggesting to cap it with
> > HASHLIMIT_MAX_SIZE too? Something like below?
> >
> > +       if (cfg->max > 2 * HASHLIMIT_MAX_SIZE)
> > +               cfg->max = 2 * HASHLIMIT_MAX_SIZE;
> >
>
> Yes, thats what I meant, cap the user-provided value to something thats
> going to be less of a problem.
>
> But now that I read it, the "2 *" part looks really silly, so I suggst
> to go with " > FOO_MAX", else its not a maximum value after all.

Ok, so here is what I have now:


+#define HASHLIMIT_MAX_SIZE 1048576
+
 static int hashlimit_mt_check_common(const struct xt_mtchk_param *par,
                                     struct xt_hashlimit_htable **hinfo,
                                     struct hashlimit_cfg3 *cfg,
@@ -847,6 +849,14 @@ static int hashlimit_mt_check_common(const struct
xt_mtchk_param *par,

        if (cfg->gc_interval == 0 || cfg->expire == 0)
                return -EINVAL;
+       if (cfg->size > HASHLIMIT_MAX_SIZE) {
+               cfg->size = HASHLIMIT_MAX_SIZE;
+               pr_info_ratelimited("size too large, truncated to
%u\n", cfg->size);
+       }
+       if (cfg->max > HASHLIMIT_MAX_SIZE) {
+               cfg->max = HASHLIMIT_MAX_SIZE;
+               pr_info_ratelimited("max too large, truncated to
%u\n", cfg->max);
+       }

Please let me know if it is still different with your suggestion.

Thanks!