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Date: Sat, 22 Apr 2017 12:29:37 -0700
From: Kees Cook <>
To: Andy Lutomirski <>
Cc: Djalal Harouni <>, 
	Linux Kernel Mailing List <>, Andrew Morton <>, 
	"Serge E. Hallyn" <>, 
	"" <>, 
	LSM List <>, 
	Linux API <>, Dongsu Park <>, 
	Casey Schaufler <>, James Morris <>, 
	Paul Moore <>, Tetsuo Handa <>, 
	Greg Kroah-Hartman <>, Jonathan Corbet <>, 
	Jessica Yu <>, Rusty Russell <>, 
	Arnaldo Carvalho de Melo <>, Mauro Carvalho Chehab <>, Ingo Molnar <>, 
	belakhdar abdeldjalil <>, Peter Zijlstra <>
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 <> wrote:
> On Fri, Apr 21, 2017 at 5:12 PM, Djalal Harouni <> wrote:
>> On Sat, Apr 22, 2017 at 1:51 AM, Andy Lutomirski <> 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?


> 3. A per-user-ns sysctl.

Kees Cook
Pixel Security

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