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

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Date:   Fri, 31 Jan 2020 18:53:21 -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 3:37 PM Florian Westphal <fw@...len.de> wrote:
> O would propose a max alloc size (hard limit) of ~8 MByte of vmalloc
> space, or maybe 16 at most.
>
> 1048576 max upperlimit -> ~8mbyte vmalloc request -> allows to store
> up to 2**23 entries.

Changing HASHLIMIT_MAX_SIZE to 1048576 seems fine.

>
> 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;

Thanks.