Re: [PATCH nf-next,RFC 08/10] netfilter: move NF_QUEUE handling away from core

[Florian Westphal <fw@strlen.de>]

Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> On Fri, Oct 14, 2016 at 04:06:15PM +0800, Liping Zhang wrote:
> > Hi Pablo,
> >
> > 2016-10-13 20:02 GMT+08:00 Pablo Neira Ayuso <pablo@netfilter.org>:
> > > +int nf_queue(struct sk_buff *skb, const struct nf_hook_state *state,
> > > +            unsigned int queuenum, bool bypass)
> > > +{
> > > +       int ret;
> > > +
> > > +       ret = __nf_queue(skb, state, queuenum);
> > > +       if (ret < 0) {
> > > +               if (ret == -ESRCH && bypass)
> > > +                       return NF_ACCEPT;
> > > +               kfree_skb(skb);
> > > +               return NF_DROP;
> > > +       }
> > > +
> > > +       return NF_STOLEN;
> >
> > I think this will break something ... Imagine such situation:
> > # ip route add default dev eth0
> > # ip rule add fwmark 0x1/0xf lookup eth1
> > # ip rule add fwmark 0x2/0xf lookup eth2
> > # iptables -t mangle -A OUTPUT -d 1.1.1.1 -j MARK --set-mark 0x1
> > # iptables -t mangle -A OUTPUT -d 2.2.2.2 -j MARK --set-mark 0x2
> > # iptables -t mangle -A OUTPUT -j NFQUEUE
> >
> > So ip packets with dst 1.1.1.1 will be sent via eth1, ip packets with
> > dst 2.2.2.2 will be sent via eth2 ...
> >
> > But apply this patch, after queue the packet with dst 1.1.1.1 to the
> > userspace and reinject it to the kernel, the packet will be sent via
> > the wrong interface, i.e. eth0 not eth1.
> >
> > Because ret is *NF_STOLEN* so we will not call ip_route_me_harder
> > to do re-route in ipt_mangle_out().
> 
> Good point. Then, we can just return NF_QUEUE here instead, which
> would become sort of an alias of NF_STOLEN, but this now just signals
> the core that the packet was enqueued to userspace. I mean:
> 
> int nf_queue(struct sk_buff *skb, const struct nf_hook_state *state,
>             unsigned int queuenum, bool bypass)
> {
>        int ret;
> 
>        ret = __nf_queue(skb, state, queuenum);
>        if (ret < 0) {
>                if (ret == -ESRCH && bypass)
>                        return NF_ACCEPT;
>                kfree_skb(skb);
>                return NF_DROP;
>        }
> 
>        return NF_QUEUE; <--- this.
> }

I'm afraid that won't fly.  When This NF_QUEUE is returned here, we're
in a race as skb is already on its way to userspace (or perhaps already
being reinjected/dropped on other cpu).

I think the simplest way out is to always re-route from nf_reinject
in case we were queued from mangle output.

For nft, we might be able to make a note of 'route' chain type in the
nf_hook_state and then have nf_reinject check for that.