linux-kernel - Re: Potential issues (security and otherwise) with the current cgroup-bpf API

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Date:   Mon, 19 Dec 2016 17:43:43 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Daniel Mack <daniel@...que.org>,
        Mickaël Salaün <mic@...ikod.net>,
        Kees Cook <keescook@...omium.org>, Jann Horn <jann@...jh.net>,
        Tejun Heo <tj@...nel.org>, David Ahern <dsahern@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Graf <tgraf@...g.ch>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: Potential issues (security and otherwise) with the current
 cgroup-bpf API

On Mon, Dec 19, 2016 at 4:25 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Dec 19, 2016 at 4:02 PM, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>> you're ignoring use cases I described earlier.
>> In vrf case there is only one ifindex it needs to bind to.
>
> I'm totally lost.  Can you explain what this has to do with the cgroup
> hierarchy?
>

Okay, I figured out what you mean, I think.  You have a handful of vrf
devices.  Let's say they have ifindexes 1 and 2 (and maybe more).

The interesting case happens when you set up /cgroup/a with a bpf
program that binds new sockets to ifindex 1 and /cgroup/a/b with a bpf
program that binds new sockets to ifindex 2.  The question is: what
should happen if you're in /cgroup/a/b?  Presumably, if you do this,
you wanted to end up with ifindex 2.

I think the way it should actually work is that the kernel evaluates
/cgroup/a/b's hook and then /cgroup/a's hook.  Then /cgroup/a (which
is the more privileged hook) gets to make the choice.  If it wants
ifindex 2 to win, it can do (pseudocode):

if (!sk->sk_bound_if)
  sk->sk_bound_if = 1;