Re: [PATCH bpf 1/2] flow_dissector: support FLOW_DISSECTOR_KEY_ETH_ADDRS with BPF

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Mon, May 13, 2019 at 3:53 PM Stanislav Fomichev <sdf@google.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.

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.

>
> 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@google.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.