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

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Date:   Fri, 21 Jun 2019 18:54:27 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Florian Westphal <fw@...len.de>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Jakub Sitnicki <jakub@...udflare.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: [RFC bpf-next 0/7] Programming socket lookup with BPF

Hi,

On Fri, 2019-06-21 at 18:41 +0200, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.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.

Perhaps I'm misreading the above, but it looks like this has some
overlapping with a past attempt:

https://marc.info/?l=linux-netdev&m=150611442802964&w=2

Cheers,

Paolo