netdev - Re: [PATCH net v2] netfilter: nat: cope with negative port range

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Date:   Wed, 14 Feb 2018 16:49:56 +0100
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Florian Westphal <fw@...len.de>,
        Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH net v2] netfilter: nat: cope with negative port range

On Wed, Feb 14, 2018 at 04:45:31PM +0100, Paolo Abeni wrote:
> Hi,
> 
> On Wed, 2018-02-14 at 14:51 +0100, Pablo Neira Ayuso wrote:
> > On Wed, Feb 14, 2018 at 01:30:37PM +0100, Florian Westphal wrote:
> > > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > > On Wed, 2018-02-14 at 12:13 +0100, Paolo Abeni wrote:
> > > > > syzbot reported a division by 0 bug in the netfilter nat code:
> > > > > Adding the relevant check at parse time could break existing
> > > > > setup, moreover we would need to read/write such values atomically
> > > > > to avoid possible transient negative ranges at update time.
> > > > 
> > > > I do not quite follow why it is so hard to add a check at parse time.
> > > > 
> > > > Breaking buggy setups would not be a concern I think.
> > > 
> > > It would be possible for xtables but afaics in nft_nat.c case
> > > (nft_nat_eval) range.{min,max}_proto.all values are loaded from nft
> > > registers at runtime.
> > 
> > Then, restrict this from nft_nat.
> 
> If we move the check in the caller for nft, then need cope individually
> with several control paths (nf_nat_setup_info() is used by ~10 modules
> if I'm not wrong), I think keeping the check here would be better, do
> you have strong opinions against that?

You're right, this is fine.

Thanks for explaining!