Re: [PATCH] nsenter: fix ability to enter unprivileged containers

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2016-04-18 at 19:11 +0200, Karel Zak wrote:
> On Mon, Apr 18, 2016 at 11:37:34AM -0400, James Bottomley wrote:
> > OK, so if you want me to reply properly, you're going to have to keep
> > my address in the cc list.
> > 
> > > > If you enter it first, you lose privilege for subsequent
> > > namespace
> > > > enters,see issue
> > > >
> > > > https://github.com/karelzak/util-linux/issues/315
> > > >
> > > > The fix is to enter the user namespace last of all.
> > > 
> > > I verified that with *current*/unpatched nsenter,
> > > 
> > > $ unshare -rm sleep inf &
> > > $ nsenter -t $! -U -m --preserve
> > > 
> > > works as expected (from regular user [and with unprivileged userns
> > > enabled]).
> > > 
> > > With this patch it *won't* work [verified], of course (as you'll need
> > > root privileges in userns before joining mount-ns, and you can only
> > > obtain them by entering userns first).
> > 
> > So we're using userns for different things.  I'm using it to remove
> > privilege (so on my userns implementation root in the host enters but
> > on becoming root in the userns, it can do nothing other than write to
> > its own files) and you're using it to enhance privilege.  It looks like
> > these two things will always be mutually exclusive, so perhaps we need
> > an extra flag to nsenter to say do the userns first or last?
> 
> That's what I have talked about at github -- see Eric's comment in the
> code, the user NS is the first in the array for a good reason. May be
> it would be really better to add --user-{first,last} options to
> specify when you want to enter user NS.

OK, I'll code this up; hang on.

James