Re: [kernel-hardening] Re: [PATCH 0/2] sysctl: allow CLONE_NEWUSER to be disabled

[Eric W. Biederman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Tue, Jan 26, 2016 at 9:15 AM, Serge Hallyn <serge.hallyn@xxxxxxxxxx> wrote:
>> Quoting Josh Boyer (jwboyer@xxxxxxxxxxxxxxxxx):
>>> What you're saying is true for the "oh crap" case of a new userns
>>> related CVE being found.  However, there is the case where sysadmins
>>> know for a fact that a set of machines should not allow user
>>> namespaces to be enabled.  Currently they have 2 choices, 1) use their
>>
>> Hi - can you give a specific example of this?  (Where users really should
>> not be able to use them - not where they might not need them)  I think
>> it'll help the discussion tremendously.  Because so far the only good
>> arguments I've seen have been about actual bugs in the user namespaces,
>> which would not warrant a designed-in permanent disable switch.  If
>> there are good use cases where such a disable switch will always be
>> needed (and compiling out can't satisfy) that'd be helpful.
>
> My example is a machine in a colo rack serving web pages. A site gets
> attacked, and www-data uses user namespaces to continue their attack
> to gain root privileges.
>
> The admin of such a machine could have disabled userns months earlier
> and limited the scope of the attack.

Of course for the paranoid there is already a mechanism to do this.
/sbin/chroot.

No new user namespaces are allowed to be created inside of a chroot.

Eric