Linux Ethernet Bridging - Aug 2019 - [Bridge] [PATCH v4 2/2] net: br_netfiler_hooks: Drops IPv6 packets if IPv6 module is not loaded

Leonardo Bras <leonardo at 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 at 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.

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.

I would go with option two.

On Fri, 2019-08-30 at 22:55 +0200, Florian Westphal
wrote:
> Leonardo Bras <leonardo at 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 at 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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Leonardo Bras <leonardo at linux.ibm.com> wrote:
> > 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?
Yes, thats the idea: Let users decide if ipv6.disable or call-ip6tables
is more important to them.