Re: [PATCH v5 0/4] Introduce security_create_user_ns()

[Serge E. Hallyn]

On Thu, Aug 25, 2022 at 01:15:46PM -0500, Eric W. Biederman wrote:
> Paul Moore <paul@xxxxxxxxxxxxxx> writes:
> 
> > On Fri, Aug 19, 2022 at 10:45 AM Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> >>  I am hoping we can come up with
> >> "something better" to address people's needs, make everyone happy, and
> >> bring forth world peace.  Which would stack just fine with what's here
> >> for defense in depth.
> >>
> >> You may well not be interested in further work, and that's fine.  I need
> >> to set aside a few days to think on this.
> >
> > I'm happy to continue the discussion as long as it's constructive; I
> > think we all are.  My gut feeling is that Frederick's approach falls
> > closest to the sweet spot of "workable without being overly offensive"
> > (*cough*), but if you've got an additional approach in mind, or an
> > alternative approach that solves the same use case problems, I think
> > we'd all love to hear about it.
> 
> I would love to actually hear the problems people are trying to solve so
> that we can have a sensible conversation about the trade offs.
> 
> As best I can tell without more information people want to use
> the creation of a user namespace as a signal that the code is
> attempting an exploit.

I don't think that's it at all.  I think the problem is that it seems
you can pretty reliably get a root shell at some point in the future
by creating a user namespace, leaving it open for a bit, and waiting
for a new announcement of the latest netfilter or whatever exploit
that requires root in a user namespace.  Then go back to your userns
shell and run the exploit.

So i was hoping we could do something more targeted.  Be it splitting
off the ability to run code under capable_ns code from uid mapping (to
an extent), or maybe some limited-livepatch type of thing where
certain parts of code become inaccessible to code in a non-init userns
after some sysctl has been toggled, or something cooloer that I've
failed to think of.

-serge