linux-kernel - Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction

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Message-ID: <CAGXu5jLV+WZyj+xnxVFkFEgEthNt6eXdcSgHT-=85mJ1ECZ1Rw@mail.gmail.com>
Date:   Sat, 22 Apr 2017 12:29:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Djalal Harouni <tixxdz@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Dongsu Park <dpark@...teo.net>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <james.l.morris@...cle.com>,
        Paul Moore <paul@...l-moore.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>, Jessica Yu <jeyu@...hat.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        belakhdar abdeldjalil <zendyani@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules
 autoload restriction

On Fri, Apr 21, 2017 at 11:51 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Fri, Apr 21, 2017 at 5:12 PM, Djalal Harouni <tixxdz@...il.com> wrote:
>> On Sat, Apr 22, 2017 at 1:51 AM, Andy Lutomirski <luto@...nel.org> wrote:
>> [...]
>>>>> I personally like my implicit_rights idea, and it might be interesting
>>>>> to prototype it.
>>>>
>>>> I don't like blocking a needed feature behind a large super-feature
>>>> that doesn't exist yet. We'd be able to refactor this code into using
>>>> such a thing in the future, so I'd prefer to move ahead with this
>>>> since it would stop actual exploits.
>>>
>>> I don't think the super-feature is so hard, and I think we should not
>>> add the per-task thing the way it's done in this patch.  Let's not add
>>> per-task things where the best argument for their security is "not
>>> sure how it would be exploited".
>>
>> Actually the XFRM framework CVE-2017-7184 [1] is one real example, of
>> course there are others. The exploit was used on a generic distro
>> during a security contest that distro is Ubuntu. That distro will
>> never provide a module autoloading restriction by default to not harm
>> it's users. Consumers or containers/sandboxes then can run their
>> confined apps using such facilities.
>>
>> These bugs will stay in embedded devices that use these generic
>> distros for ever.
>>
>>> Anyway, I think the sysctl is really the important bit.  The per-task
>>> setting is icing on the cake IMO.  One upon a time autoload was more
>>> important, but these days modaliases are supposed to do most of the
>>> work.  I bet that modern distros don't need unprivileged autoload at
>>> all.
>>
>> Actually I think they do and we can't just change that. Users may
>> depend on it, it is a well established facility.
>>
>> Now the other problem is CAP_NET_ADMIN which does lot of things, it is
>> more like the CAP_SYS_ADMIN.
>>
>> This is a quick list that I got from only the past months, I'm pretty
>> sure there are more:
>>
>> * DCCP use after free CVE-2017-6074
>> * n_hldc CVE-2017-2636
>> * XFRM framework CVE-2017-7184
>> * L2TPv3 CVE-2016-10200
>>
>> Most of these need CAP_NET_ADMIN to be autoloaded, however we also
>> need CAP_NET_ADMIN for other things... therefore it is better to have
>> an extra facility that could coexist with CAP_NET_ADMIN and other
>> sandbox features.
>>
>
> I agree that the feature is important, but I think your implementation
> is needlessly dangerous.  I imagine that the main uses that you care
> about involve containers.  How about doing it in a safer way that
> works for containers?  I can think of a few.  For example:
>
> 1. A sysctl that, if set, prevents autoloading outside the root
> userns.  This isn't very flexible at all, but it might work.
>
> 2. Your patch, but require privilege within the calling namespace to
> set the prctl.

How about CAP_SYS_ADMIN || no_new_privs?

-Kees

>
> 3. A per-user-ns sysctl.



-- 
Kees Cook
Pixel Security