netdev - Re: [PATCH v4 2/2] net: br_netfiler_hooks: Drops IPv6 packets if IPv6 module is not loaded

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Date:   Sat, 31 Aug 2019 01:42:29 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Florian Westphal <fw@...len.de>
Cc:     netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        bridge@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v4 2/2] net: br_netfiler_hooks: Drops IPv6 packets if
 IPv6 module is not loaded

On Fri, 2019-08-30 at 22:55 +0200, Florian Westphal wrote:
> Leonardo Bras <leonardo@...ux.ibm.com> wrote:
> > A kernel panic can happen if a host has disabled IPv6 on boot and have to
> > process guest packets (coming from a bridge) using it's ip6tables.
> > 
> > IPv6 packets need to be dropped if the IPv6 module is not loaded.
> > 
> > Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
> > ---
> >  net/bridge/br_netfilter_hooks.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
> > index d3f9592f4ff8..5e8693730df1 100644
> > --- a/net/bridge/br_netfilter_hooks.c
> > +++ b/net/bridge/br_netfilter_hooks.c
> > @@ -493,6 +493,8 @@ static unsigned int br_nf_pre_routing(void *priv,
> >  	brnet = net_generic(state->net, brnf_net_id);
> >  	if (IS_IPV6(skb) || is_vlan_ipv6(skb, state->net) ||
> >  	    is_pppoe_ipv6(skb, state->net)) {
> > +		if (!ipv6_mod_enabled())
> > +			return NF_DROP;
> >  		if (!brnet->call_ip6tables &&
> >  		    !br_opt_get(br, BROPT_NF_CALL_IP6TABLES))
> >  			return NF_ACCEPT;
> 
> No, thats too aggressive and turns the bridge into an ipv6 blackhole.
> 
> There are two solutions:
> 1. The above patch, but use NF_ACCEPT instead
> 2. keep the DROP, but move it below the call_ip6tables test,
>    so that users can tweak call-ip6tables to accept packets.

Q: Does 2 mean that it will only be dropped if bridge intents to use
host's ip6tables? Else, it will be accepted by previous if?

> Perhaps it would be good to also add a pr_warn_once() that
> tells that ipv6 was disabled on command line and
> call-ip6tables isn't supported in this configuration.
> 
Good idea, added.

> I would go with option two.
I think it's better than 1 too.

I sent a v5 with these changes:
https://lkml.org/lkml/2019/8/31/4

Thanks!

Leonardo Bras

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