Re: [PATCH] neighbour: purge nf_bridged skb from foreign device neigh

[Pavel Tikhomirov]

Here is the new version 
https://lore.kernel.org/netdev/20240110110451.5473-1-ptikhomirov@xxxxxxxxxxxxx/

On 09/01/2024 19:12, Florian Westphal wrote:
> Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx> wrote:
> > index f980edfdd2783..105fbdb029261 100644
> > --- a/include/linux/netfilter_bridge.h
> > +++ b/include/linux/netfilter_bridge.h
> > @@ -56,11 +56,15 @@ static inline int nf_bridge_get_physoutif(const struct
> > sk_buff *skb)
> >   }
> >
> >   static inline struct net_device *
> > -nf_bridge_get_physindev(const struct sk_buff *skb)
> > +nf_bridge_get_physindev_rcu(const struct sk_buff *skb)
> >   {
> >          const struct nf_bridge_info *nf_bridge = nf_bridge_info_get(skb);
> > +       struct net_device *dev;
> >
> > -       return nf_bridge ? nf_bridge->physindev : NULL;
> > +       if (!nf_bridge || !skb->dev)
> > +               return 0;
> > +
> > +       return dev_get_by_index_rcu(skb->dev->net, nf_bridge->physindev_if);
>
> You could use dev_net(skb->dev), yes.

In br_nf_pre_routing_finish_bridge_slow I had to use dev_net(skb->dev).

> Or create a preparation patch that does:
>
> -nf_bridge_get_physindev(const struct sk_buff *skb)
> +nf_bridge_get_physindev(const struct sk_buff *skb, struct net *net)
>
> (all callers have a struct net available).

For all other cases I did the prep-patch propagating net.

> No need to rename the function, see below.
>
> > -       br_indev = nf_bridge_get_physindev(oldskb);
> > +       rcu_read_lock_bh();
> > +       br_indev = nf_bridge_get_physindev_rcu(oldskb);
>
> No need for rcu read lock, all netfilter hooks run inside
> rcu_read_lock().

Thanks for this hint! I have checked all those tons of cases and 
actually proved to myself that all cases have rcu_read_lock =)

> > Does it sound good?
>
> Yes, seems ok to me.
>
> > Or maybe instead we can have extra physindev_if field in addition to
> > existing physindev to only do dev_get_by_index_rcu inside
> > br_nf_pre_routing_finish_bridge_slow to doublecheck the ->physindev link?
> >
> > Sorry in advance if I'm missing anything obvious.
>
> Alternative would be to add a 'br_nf_unreg_serno' that gets incremented
> from brnf_device_event(), then store that in nf_bridge_info struct and
> compare to current value before net_device deref. If not equal, toss skb.
>
> Problem is that we'd need some indirection to retrieve the current
> value, otherwise places like nfnetlink_log() gain a module dependency on
> br_netfilter :-(
>
> We'd likely need
> const atomic_t *br_nf_unreg_serno __read_mostly;
> EXPORT_SYMBOL_GPL(br_nf_unreg_serno);
>
> in net/netfilter/core.c for this, then set/clear the
> pointer from br_netfilter_hooks.c.
>
> I can't say/don't know which of the two options is better/worse.
>
> s/struct net_device */int// has the benefit of shrinking nf_bridge_info,
> so I'd try that first.

Ok, did s/struct net_device */int// variant.

--
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.