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

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Date:   Fri, 31 Jan 2020 15:16:44 -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 Fri, Jan 31, 2020 at 2:08 PM Florian Westphal <fw@...len.de> wrote:
>
> Cong Wang <xiyou.wangcong@...il.com> wrote:
> > The user-specified hashtable size is unbound, this could
> > easily lead to an OOM or a hung task as we hold the global
> > mutex while allocating and initializing the new hashtable.
> >
> > The max value is derived from the max value when chosen by
> > the kernel.
> >
> > Reported-and-tested-by: syzbot+adf6c6c2be1c3a718121@...kaller.appspotmail.com
> > Cc: Pablo Neira Ayuso <pablo@...filter.org>
> > Cc: Jozsef Kadlecsik <kadlec@...filter.org>
> > Cc: Florian Westphal <fw@...len.de>
> > Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> > ---
> >  net/netfilter/xt_hashlimit.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
> > index 57a2639bcc22..6327134c5886 100644
> > --- a/net/netfilter/xt_hashlimit.c
> > +++ b/net/netfilter/xt_hashlimit.c
> > @@ -272,6 +272,8 @@ dsthash_free(struct xt_hashlimit_htable *ht, struct dsthash_ent *ent)
> >  }
> >  static void htable_gc(struct work_struct *work);
> >
> > +#define HASHLIMIT_MAX_SIZE 8192
> > +
> >  static int htable_create(struct net *net, struct hashlimit_cfg3 *cfg,
> >                        const char *name, u_int8_t family,
> >                        struct xt_hashlimit_htable **out_hinfo,
> > @@ -290,7 +292,7 @@ static int htable_create(struct net *net, struct hashlimit_cfg3 *cfg,
> >               size = (nr_pages << PAGE_SHIFT) / 16384 /
> >                      sizeof(struct hlist_head);
> >               if (nr_pages > 1024 * 1024 * 1024 / PAGE_SIZE)
> > -                     size = 8192;
> > +                     size = HASHLIMIT_MAX_SIZE;
> >               if (size < 16)
> >                       size = 16;
> >       }
> > @@ -848,6 +850,8 @@ 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)
> > +             return -ENOMEM;
>
> Hmm, won't that break restore of rulesets that have something like
>
> --hashlimit-size 10000?
>
> AFAIU this limits the module to vmalloc requests of only 64kbyte.
> I'm not opposed to a limit (or a cap), but 64k seems a bit low to me.

8192 is from the current code which handles kernel-chosen size
(that is cfg->size==0), I personally have no idea what the max
should be. :)

Please suggest a number.

Thanks.