Re: Thoughts on tightening up user namespace creation

From: Austin S. Hemmelgarn
Date: Wed Mar 09 2016 - 14:05:40 EST

On 2016-03-09 13:51, Colin Walters wrote:
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:

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
writing setuid binaries that expose some of the features, but e.g. not
At least Google Chrome (and probably Chromium) is using user namespaces without CAP_SYS_ADMIM (although AFAIUI, it's because they can't use the other namespace types effectively as a regular user).

Andy's suggestion of having this be a per-namespace setting makes
sense to me. Currently some container tools that do use userns
are by default denying it to be recursive ( and Docker 1.10 at least)
by using a seccomp filter on clone(). If we had this setting that
filter wouldn't be necessary, and would solve the issue that seccomp filters
aren't robust against the kernel adding new API, e.g. a new CLONE_NEWUSER_NONEWPRIVS
which might enable chroot() but not CAP_NET_ADMIN.

Personally, I like the suggestion from Alexander Larsson to make a cgroup controller. Container tools obviously want some degree of hierarchical control (even if it's just saying that the hierarchy ends here), and it would simplify the possibility of running more than one container stack on the same host (I know at least a couple people who would love to be able to safely use Docker on the same host as LXC or lmctfy).