Re: Thoughts on tightening up user namespace creation

[Kees Cook]

On Wed, Mar 9, 2016 at 11:21 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Colin Walters (walters@xxxxxxxxxx):
>> On Wed, Mar 9, 2016, at 01:14 PM, Kees Cook wrote:
>> > On Mon, Mar 7, 2016 at 9:15 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> > > Hi all-
>> > >
>> > > There are several users and distros that are nervous about user
>> > > namespaces from an attack surface point of view.
>> > >
>> > >  - RHEL and Arch have userns disabled.
>> > >
>> > >  - Ubuntu requires CAP_SYS_ADMIN
>> > >
>> > >  - Kees periodically proposes to upstream some sysctl to control
>> > > userns creation.
>> >
>> > And here's another ring0 escalation flaw, made available to
>> > unprivileged users because of userns:
>> >
>> > https://code.google.com/p/google-security-research/issues/detail?id=758
>>
>> Looks like Andy won't have to eat his hat ;)
>>
>> > The change in attack surface is _substantial_. We must have a way to
>> > globally disable userns.
>>
>> No one would object if it was enabled but only accessible to
>> CAP_SYS_ADMIN though, right?  This could be useful for
>
> I think that would be terrible.  I'd have to expose all of CAP_SYS_ADMIN
> to allow use of CLONE_NEWUSER.  I'd be more interested in a new CAP_NEWUSER
> capability.  Then systems wanting to support unprivileged users doing user
> namespaces could set a pam module giving certain users that cap in pI, and
> set it on fI on their container managers.  Userspace has to give access to
> mapped uids through /etc/subuid too, so it's not *so* huge added hurdle.
> Well that's not quite true - with empty subuid, users can create a userns
> with no mapped userids which in itself is useful for sandboxing.
>
> The biggest problem with a CAP_NEWUSER would be that it's more inherently
> permanent than a new sysctl.  The increase in attack surface is real, but
> over time I'd like to think that we will have dealt with it and should be
> able to make CLONE_NEWUSER unprivileged.  Because what we have is an
> implementation issue (not in user namespaces), not a design issue.

Andy suggested a capability back in October. But I agree with you, we
don't want a new capability. https://lkml.org/lkml/2015/10/17/94

> And I do agree the issue is real.

And I fully expect for the issue to improve over time: it's not that I
don't want userns, I just want to have the _option_ to disable it at
runtime for the systems that don't need it until the newly exposed
interfaces look like they've had the bulk of their issues resolved.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security