netdev - Re: [bpf-next RFC 1/3] flow_dissector: implements flow dissector BPF hook

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Date:   Mon, 20 Aug 2018 10:13:42 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     liu.song.a23@...il.com
Cc:     Petar Penkov <ppenkov@...gle.com>,
        Petar Penkov <peterpenkov96@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        simon.horman@...ronome.com, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [bpf-next RFC 1/3] flow_dissector: implements flow dissector BPF hook

On Mon, Aug 20, 2018 at 1:47 AM Song Liu <liu.song.a23@...il.com> wrote:
>
> On Thu, Aug 16, 2018 at 4:14 PM, Petar Penkov <ppenkov@...gle.com> wrote:
> > On Thu, Aug 16, 2018 at 3:40 PM, Song Liu <liu.song.a23@...il.com> wrote:
> >>
> >> On Thu, Aug 16, 2018 at 9:44 AM, Petar Penkov <peterpenkov96@...il.com> wrote:
> >> > From: Petar Penkov <ppenkov@...gle.com>
> >> >
> >> > Adds a hook for programs of type BPF_PROG_TYPE_FLOW_DISSECTOR and
> >> > attach type BPF_FLOW_DISSECTOR that is executed in the flow dissector
> >> > path. The BPF program is kept as a global variable so it is
> >> > accessible to all flow dissectors.
> >> >
> >> > Signed-off-by: Petar Penkov <ppenkov@...gle.com>
> >> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>

> >> > @@ -658,6 +698,42 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
> >> >                                               FLOW_DISSECTOR_KEY_BASIC,
> >> >                                               target_container);
> >> >
> >> > +       rcu_read_lock();
> >> > +       attached = rcu_dereference(flow_dissector_prog);
> >> > +       if (attached) {
> >> > +               /* Note that even though the const qualifier is discarded
> >> > +                * throughout the execution of the BPF program, all changes(the
> >> > +                * control block) are reverted after the BPF program returns.
> >> > +                * Therefore, __skb_flow_dissect does not alter the skb.
> >> > +                */
> >> > +               struct bpf_flow_dissect_cb *cb;
> >> > +               u8 cb_saved[BPF_SKB_CB_LEN];
> >> > +               u32 result;
> >> > +
> >> > +               cb = (struct bpf_flow_dissect_cb *)(bpf_skb_cb((struct sk_buff *)skb));
> >> > +
> >> > +               /* Save Control Block */
> >> > +               memcpy(cb_saved, cb, sizeof(cb_saved));
> >> > +               memset(cb, 0, sizeof(cb_saved));
> >> > +
> >> > +               /* Pass parameters to the BPF program */
> >> > +               cb->nhoff = nhoff;
> >> > +               cb->target_container = target_container;
> >> > +               cb->flow_dissector = flow_dissector;
> >> > +
> >> > +               bpf_compute_data_pointers((struct sk_buff *)skb);
> >> > +               result = BPF_PROG_RUN(attached, skb);
> >> > +
> >> > +               /* Restore state */
> >> > +               memcpy(cb, cb_saved, sizeof(cb_saved));
> >> > +
> >> > +               key_control->thoff = min_t(u16, key_control->thoff,
> >> > +                                          skb ? skb->len : hlen);
> >> > +               rcu_read_unlock();
> >> > +               return result == BPF_OK;
> >> > +       }
> >>
> >> If the BPF program cannot handle certain protocol, shall we fall back
> >> to the built-in logic? Otherwise, all BPF programs need to have some
> >> code for all protocols.
> >>
> >> Song
> >
> > I believe that if we fall back to the built-in logic we lose all security
> > guarantees from BPF and this is why the code does not support
> > fall back.
> >
> > Petar
>
> I am not really sure we are on the same page. I am proposing 3
> different return values from BPF_PROG_RUN(), and they should be
> handled as
>
> 1. result == BPF_OK                      => return true;
> 2. result == BPF_DROP                 => return false;
> 3. result == something else            => fall back.
>
> Does this proposal make any sense?
>
> Thanks,
> Song

It certainly makes sense. We debated it initially, as well.

In the short term, it allows for simpler BPF programs, as they can
off-load some protocols to the C implementation.

But the RFC patchset already implements most protocols in BPF.
I had not expected that when we started out.

Eventually, I think it is preferable to just deprecate the C
implementation. Which is not possible if we make this opt-out
a part of the BPF flow dissector interface.

There is also the lesser issue that a buggy BPF program might
accidentally pass the third value and unknowing open itself up
to the large attack surface. Without this option, the security
audit is much simpler.