netdev - Re: [PATCH bpf 1/2] flow_dissector: support FLOW_DISSECTOR_KEY_ETH

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Date:   Mon, 13 May 2019 17:21:48 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>
Subject: Re: [PATCH bpf 1/2] flow_dissector: support FLOW_DISSECTOR_KEY_ETH_ADDRS
 with BPF

On Mon, May 13, 2019 at 5:02 PM Stanislav Fomichev <sdf@...ichev.me> wrote:
>
> On 05/13, Willem de Bruijn wrote:
> > On Mon, May 13, 2019 at 3:53 PM Stanislav Fomichev <sdf@...gle.com> wrote:
> > >
> > > If we have a flow dissector BPF program attached to the namespace,
> > > FLOW_DISSECTOR_KEY_ETH_ADDRS won't trigger because we exit early.
> >
> > I suppose that this is true for a variety of keys? For instance, also
> > FLOW_DISSECTOR_KEY_IPV4_ADDRS.

> I though the intent was to support most of the basic stuff (eth/ip/tcp/udp)
> without any esoteric protocols.

Indeed. But this applies both to protocols and the feature set. Both
are more limited.

> Not sure about FLOW_DISSECTOR_KEY_IPV4_ADDRS,
> looks like we support that (except FLOW_DISSECTOR_KEY_TIPC part).

Ah, I chose a bad example then.

> > We originally intended BPF flow dissection for all paths except
> > tc_flower. As that catches all the vulnerable cases on the ingress
> > path on the one hand and it is infeasible to support all the
> > flower features, now and future. I think that is the real fix.

> Sorry, didn't get what you meant by the real fix.
> Don't care about tc_flower? Just support a minimal set of features
> needed by selftests?

I do mean exclude BPF flow dissector (only) for tc_flower, as we
cannot guarantee that the BPF program can fully implement the
requested feature.

>
> > >
> > > Handle FLOW_DISSECTOR_KEY_ETH_ADDRS before BPF and only if we have
> > > an skb (used by tc-flower only).
> > >
> > > Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > ---
> > >  net/core/flow_dissector.c | 23 ++++++++++++-----------
> > >  1 file changed, 12 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> > > index 9ca784c592ac..ba76d9168c8b 100644
> > > --- a/net/core/flow_dissector.c
> > > +++ b/net/core/flow_dissector.c
> > > @@ -825,6 +825,18 @@ bool __skb_flow_dissect(const struct net *net,
> > >                         else if (skb->sk)
> > >                                 net = sock_net(skb->sk);
> > >                 }
> > > +
> > > +               if (dissector_uses_key(flow_dissector,
> > > +                                      FLOW_DISSECTOR_KEY_ETH_ADDRS)) {
> > > +                       struct ethhdr *eth = eth_hdr(skb);
> >
> > Here as well as in the original patch: is it safe to just cast to
> > eth_hdr? In the same file, __skb_flow_dissect_gre does test for
> > (encapsulated) protocol first.

> Good question, I guess the assumption here is that
> FLOW_DISSECTOR_KEY_ETH_ADDRS is only used by tc_flower and the appropriate
> checks should be there as well.
> It's probably better to check skb->proto here though.

Right, as a mistaken or malicious admin can request it on a non
Ethernet device and read garbage.