Re: [PATCH bpf-next v2 1/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Oct 4, 2019 at 8:58 AM Stanislav Fomichev <sdf@google.com> wrote:
>
> Always use init_net flow dissector BPF program if it's attached and fall
> back to the per-net namespace one. Also, deny installing new programs if
> there is already one attached to the root namespace.
> Users can still detach their BPF programs, but can't attach any
> new ones (-EEXIST).
>
> Cc: Petar Penkov <ppenkov@google.com>
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> ---

Looks good, but see my note below. Regardless:

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  Documentation/bpf/prog_flow_dissector.rst |  3 ++
>  net/core/flow_dissector.c                 | 42 ++++++++++++++++++++---
>  2 files changed, 41 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/bpf/prog_flow_dissector.rst b/Documentation/bpf/prog_flow_dissector.rst
> index a78bf036cadd..4d86780ab0f1 100644
> --- a/Documentation/bpf/prog_flow_dissector.rst
> +++ b/Documentation/bpf/prog_flow_dissector.rst
> @@ -142,3 +142,6 @@ BPF flow dissector doesn't support exporting all the metadata that in-kernel
>  C-based implementation can export. Notable example is single VLAN (802.1Q)
>  and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys``
>  for a set of information that's currently can be exported from the BPF context.
> +
> +When BPF flow dissector is attached to the root network namespace (machine-wide
> +policy), users can't override it in their child network namespaces.
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 7c09d87d3269..9821e730fc70 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -114,19 +114,50 @@ int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
>  {
>         struct bpf_prog *attached;
>         struct net *net;
> +       int ret = 0;
>
>         net = current->nsproxy->net_ns;
>         mutex_lock(&flow_dissector_mutex);
> +
> +       if (net == &init_net) {
> +               /* BPF flow dissector in the root namespace overrides
> +                * any per-net-namespace one. When attaching to root,
> +                * make sure we don't have any BPF program attached
> +                * to the non-root namespaces.
> +                */
> +               struct net *ns;
> +
> +               for_each_net(ns) {
> +                       if (net == &init_net)
> +                               continue;

You don't need this condition, if something is attached to init_net,
you will return -EEXIST anyway. Or is this a performance optimization?

> +
> +                       if (rcu_access_pointer(ns->flow_dissector_prog)) {
> +                               ret = -EEXIST;
> +                               goto out;
> +                       }
> +               }
> +       } else {
> +               /* Make sure root flow dissector is not attached
> +                * when attaching to the non-root namespace.
> +                */
> +

nit: extra empty line

> +               if (rcu_access_pointer(init_net.flow_dissector_prog)) {
> +                       ret = -EEXIST;
> +                       goto out;
> +               }
> +       }
> +
>         attached = rcu_dereference_protected(net->flow_dissector_prog,
>                                              lockdep_is_held(&flow_dissector_mutex));
>         if (attached) {
>                 /* Only one BPF program can be attached at a time */
> -               mutex_unlock(&flow_dissector_mutex);
> -               return -EEXIST;
> +               ret = -EEXIST;
> +               goto out;
>         }
>         rcu_assign_pointer(net->flow_dissector_prog, prog);
> +out:
>         mutex_unlock(&flow_dissector_mutex);
> -       return 0;
> +       return ret;
>  }
>
>  int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
> @@ -910,7 +941,10 @@ bool __skb_flow_dissect(const struct net *net,
>         WARN_ON_ONCE(!net);
>         if (net) {
>                 rcu_read_lock();
> -               attached = rcu_dereference(net->flow_dissector_prog);
> +               attached = rcu_dereference(init_net.flow_dissector_prog);
> +
> +               if (!attached)
> +                       attached = rcu_dereference(net->flow_dissector_prog);
>
>                 if (attached) {
>                         struct bpf_flow_keys flow_keys;
> --
> 2.23.0.581.g78d2f28ef7-goog
>