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Date: Mon, 19 Dec 2016 21:32:57 -0800
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: David Ahern <dsahern@gmail.com>, Andy Lutomirski <luto@kernel.org>,
        Daniel Mack <daniel@zonque.org>,
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        Linux API <linux-api@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>
Subject: Re: Potential issues (security and otherwise) with the current
 cgroup-bpf API
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On Mon, Dec 19, 2016 at 09:27:18PM -0800, Andy Lutomirski wrote:
> On Mon, Dec 19, 2016 at 8:44 PM, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> > On Mon, Dec 19, 2016 at 07:12:48PM -0800, Andy Lutomirski wrote:
> >>
> >> struct cgroup_bpf {
> >>         /*
> >>          * Store two sets of bpf_prog pointers, one for programs that are
> >>          * pinned directly to this cgroup, and one for those that are effective
> >>          * when this cgroup is accessed.
> >>          */
> >>         struct bpf_prog *prog[MAX_BPF_ATTACH_TYPE];
> >>         struct bpf_prog *effective[MAX_BPF_ATTACH_TYPE];
> >> };
> >>
> >> in struct cgroup, there's a 'struct cgroup_bpf bpf;'.
> >>
> >> This would change to something like:
> >>
> >> struct cgroup_filter_slot {
> >>   struct bpf_prog *effective;
> >>   struct cgroup_filter_slot *next;
> >>   struct bpf_prog *local;
> >> }
> >>
> >> local is NULL unless *this* cgroup has a filter.  effective points to
> >> the bpf_prog that's active in this cgroup or the nearest ancestor that
> >> has a filter.  next is NULL if there are no filters higher in the
> >> chain or points to the next slot that has a filter.  struct cgroup
> >> has:
> >>
> >> struct cgroup_filter_slot filters[MAX_BPF_ATTACH_TYPE];
> >>
> >> To evaluate it, you do:
> >>
> >> struct cgroup_filter_slot *slot = &cgroup->slot[the index];
> >>
> >> if (!slot->effective)
> >>   return;
> >>
> >> do {
> >>   evaluate(slot->effective);
> >>   slot = slot->next;
> >> } while (unlikely(slot));
> >
> > yes. something like this can work as a future extension
> > to support multiple programs for security use case.
> > Please propose a patch.
> > Again, it's not needed today and there is no rush to implement it.
> >
> 
> If this happens after 4.10 and 4.10 is released as is, then this
> change would be an ABI break.

it won't break existing apps.
please study how bpf syscall was extended in the past without
breaking anything.
Same thing here. The default is one program per hook per cgroup.
Everything else is future extensions.