Quoting James Morris (james.l.morris-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org):

On Mon, 8 Jan 2018, Serge E. Hallyn wrote:

quoted

quoted

Also, why do we need the concept of a controlled user-ns at all, if the 
default whitelist maintains existing behavior?

In past discussions two uses have been brought up:

1. if an 0-day is discovered which is exacerbated by a specific
privilege in user namespaces, that privilege could be turned off until a
reboot with a fixed kernel is scheduled, without fully disabling all
containers.

2. some systems may be specifically designed to run software which
only requires a few capabilities in a userns.  In that case all others
could be disabled.

I meant in terms of "marking" a user ns as "controlled" type -- it's 
unnecessary jargon from an end user point of view.

Ah, yes, that was my point in

http://lkml.iu.edu/hypermail/linux/kernel/1711.1/01845.html
and
http://lkml.iu.edu/hypermail/linux/kernel/1711.1/02276.html

This may happen internally but don't make it a special case with a 
different name and don't bother users with internal concepts: simply 
implement capability whitelists with the default having equivalent 
behavior of everything allowed.  Then, document the semantics of the 
whitelist in terms of inheritance etc., as a feature of user namespaces, 
not as a "type" of user namespace.

The problem with making them inheritable is that an adversarial user
can just create a user namespace at boot that sits and waits for an
0day to be published, then log in and attach to that namespace later,
since it has already inherited the open whitelist.

It feels like there must be some other approach that doesn't feel as...
band-aid-y as this does, but I'm not sure what.