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

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Message-ID: <57D9CB25.1010103@digikod.net>
Date:   Thu, 15 Sep 2016 00:11:49 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "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 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?

> 
> I have no problem with looking at prototypes for how landlock +
> cgroups would work, but I can't imagine the result being mergeable.
> 



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