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

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Message-ID: <20170117133204.GA6515@twins.programming.kicks-ass.net>
Date:   Tue, 17 Jan 2017 14:32:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Tejun Heo <tj@...nel.org>, Andy Lutomirski <luto@...capital.net>,
        David Ahern <dsahern@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Graf <tgraf@...g.ch>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        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 Tue, Jan 17, 2017 at 02:03:03PM +0100, Michal Hocko wrote:
> On Sun 15-01-17 20:19:01, Tejun Heo wrote:
> [...]
> > So, what's proposed is a proper part of bpf.  In terms of
> > implementation, cgroup helps by hosting the pointers but that doesn't
> > necessarily affect the conceptual structure of it.  Given that, I
> > don't think it'd be a good idea to add anything to cgroup interface
> > for this feature.  Introspection is great to have but this should be
> > introspectable together with other bpf programs using the same
> > mechanism.  That's where it belongs.
> 
> If BPF only piggy backs on top of cgroup to iterate tasks shouldn't we
> at least enforce that the cgroup has to be a leaf one and no further
> children groups can be created once there is BPF program attached?

Why (again) this stupid constraint?

If you want to use cgroups for tagging (like perf does), _any_ parent
cgroup will also tag you.

So creating child cgroups, and placing tasks in it, should not be a
problem, the BPF thing should apply to all of them.