Re: [RFC bpf-next 0/7] Programming socket lookup with BPF

[Florian Westphal <fw@strlen.de>]

Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > AFAICS so far this would be enough:
> > 
> > 1. remove the BUG_ON() in skb_orphan, letting it clear skb->sk instead
> > 2. in nf_queue_entry_get_refs(), if skb->sk and no destructor:
> >    call nf_tproxy_assign_sock() so a reference gets taken.
> > 3. change skb_steal_sock:
> >    static inline struct sock *skb_steal_sock(struct sk_buff *skb, bool *refcounted)
> >     [..]
> >     *refcounted = skb->destructor != NULL;
> > 4. make tproxy sk assign elide the destructor assigment in case of
> >    a listening sk.
> > 
> 
> Okay, but how do we make sure the skb->sk association does not leak from rcu section ?

From netfilter pov the only escape point is nfqueue (and kfree_skb),
so for tcp/udp it will end up in their respective rx path eventually.
But you are right in that we need to also audit all NF_STOLEN users that
can be invoked from PRE_ROUTING and INPUT hooks.

OUTPUT/FORWARD/POSTROUTING are not relevant, in case skb enters IP forwarding,
it will be dropped there (we have a check to toss skb with socket
attached in forward).

In recent hallway discussion Eric suggested to add a empty destructor
stub, it would allow to do the needed annotation, i.e.
no need to change skb_orphan(), *refcounted would be set via
skb->destructor != noref_listen_skb_destructor check.

> Note we have the noref/refcounted magic for skb_dst(), we might try to use something similar
> for skb->sk

Yes, would be more code churn because we have to replace skb->sk access
by a helper to mask off NOREF bit (or we need to add a "noref" bit in
sk_buff itself).