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Date:   Fri, 30 Aug 2019 22:55:41 +0200
From:   Florian Westphal <fw@strlen.de>
To:     Leonardo Bras <leonardo@linux.ibm.com>
Cc:     netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
        bridge@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Pablo Neira Ayuso <pablo@netfilter.org>,
        Jozsef Kadlecsik <kadlec@netfilter.org>,
        Florian Westphal <fw@strlen.de>,
        Roopa Prabhu <roopa@cumulusnetworks.com>,
        Nikolay Aleksandrov <nikolay@cumulusnetworks.com>,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v4 2/2] net: br_netfiler_hooks: Drops IPv6 packets if
 IPv6 module is not loaded
Message-ID: <20190830205541.GR20113@breakpoint.cc>
References: <20190830181354.26279-1-leonardo@linux.ibm.com>
 <20190830181354.26279-3-leonardo@linux.ibm.com>
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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.

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.