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

Leonardo Bras
2019-08-31T04:42:50Z

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On Fri, 2019-08-30 at 22:55 +0200, Florian Westphal wrote:
> Leonardo Bras <leonardo@linux.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@linux.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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