linux-kernel - Re: [RFC v3 18/22] cgroup,landlock: Add CGRP_NO_NEW

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Date:   Wed, 14 Sep 2016 19:19:41 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Mack <daniel@...que.org>,
        David Drysdale <drysdale@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        James Morris <james.l.morris@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Paul Moore <pmoore@...hat.com>,
        Sargun Dhillon <sargun@...gun.me>,
        "Serge E . Hallyn" <serge@...lyn.com>, Tejun Heo <tj@...nel.org>,
        Will Drewry <wad@...omium.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>
Subject: Re: [RFC v3 18/22] cgroup,landlock: Add CGRP_NO_NEW_PRIVS to handle
 unprivileged hooks

On Wed, Sep 14, 2016 at 06:25:07PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 14, 2016 at 3:11 PM, Mickaël Salaün <mic@...ikod.net> wrote:
> >
> > On 14/09/2016 20:27, Andy Lutomirski wrote:
> >> On Wed, Sep 14, 2016 at 12:24 AM, Mickaël Salaün <mic@...ikod.net> wrote:
> >>> Add a new flag CGRP_NO_NEW_PRIVS for each cgroup. This flag is initially
> >>> set for all cgroup except the root. The flag is clear when a new process
> >>> without the no_new_privs flags is attached to the cgroup.
> >>>
> >>> If a cgroup is landlocked, then any new attempt, from an unprivileged
> >>> process, to attach a process without no_new_privs to this cgroup will
> >>> be denied.
> >>
> >> Until and unless everyone can agree on a way to properly namespace,
> >> delegate, etc cgroups, I think that trying to add unprivileged
> >> semantics to cgroups is nuts.  Given the big thread about cgroup v2,
> >> no-internal-tasks, etc, I just don't see how this approach can be
> >> viable.
> >
> > As far as I can tell, the no_new_privs flag of at task is not related to
> > namespaces. The CGRP_NO_NEW_PRIVS flag is only a cache to quickly access
> > the no_new_privs property of *tasks* in a cgroup. The semantic is unchanged.
> >
> > Using cgroup is optional, any task could use the seccomp-based
> > landlocking instead. However, for those that want/need to manage a
> > security policy in a more dynamic way, using cgroups may make sense.
> >
> > I though cgroup delegation was OK in the v2, isn't it the case? Do you
> > have some links?
> >
> >>
> >> Can we try to make landlock work completely independently of cgroups
> >> so that it doesn't get stuck and so that programs can use it without
> >> worrying about cgroup v1 vs v2, interactions with cgroup managers,
> >> cgroup managers that (supposedly?) will start migrating processes
> >> around piecemeal and almost certainly blowing up landlock in the
> >> process, etc?
> >
> > This RFC handle both cgroup and seccomp approaches in a similar way. I
> > don't see why building on top of cgroup v2 is a problem. Is there
> > security issues with delegation?
> 
> What I mean is: cgroup v2 delegation has a functionality problem.
> Tejun says [1]:
> 
> We haven't had to face this decision because cgroup has never properly
> supported delegating to applications and the in-use setups where this
> happens are custom configurations where there is no boundary between
> system and applications and adhoc trial-and-error is good enough a way
> to find a working solution.  That wiggle room goes away once we
> officially open this up to individual applications.
> 
> Unless and until that changes, I think that landlock should stay away
> from cgroups.  Others could reasonably disagree with me.

Ours and Sargun's use cases for cgroup+lsm+bpf is not for security
and not for sandboxing. So the above doesn't matter in such contexts.
lsm hooks + cgroups provide convenient scope and existing entry points.
Please see checmate examples how it's used.