[PATCH 11/11] newuidmap, newgidmap: New suid helpers for using subordinate uids and gids

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> Serge Hallyn <serge.hallyn at ubuntu.com> writes:
> 
> > Quoting Eric W. Biederman (ebiederm at xmission.com):
> >> "Serge E. Hallyn" <serge at hallyn.com> writes:
> >> 
> >> > Quoting Eric W. Biederman (ebiederm at xmission.com):
> >> >
> >> > Hi,
> >> >
> >> >> +static bool verify_range(struct passwd *pw, struct map_range *range)
> >> >> +{
> >> >> +	/* An empty range is invalid */
> >> >> +	if (range->count == 0)
> >> >> +		return false;
> >> >> +
> >> >> +	/* Test /etc/subuid */
> >> >> +	if (have_sub_uids(pw->pw_name, range->lower, range->count))
> >> >> +		return true;
> >> >
> >> > I think the have_sub_uids() test should be skipped if we started
> >> > out as root.  Is there a reason not to do that?
> >> 
> >> The only reason I can see for root to use this binary is if it a flavor
> >> of root that has dropped some capbilities.  Is there a reason for root
> >> to use newuidmap and newgid map at all?
> >
> > Of course.  It keeps things simpler for creating mapped containers.
> > Otherwise we have to special-case the "i am root" vs "i am not root"
> > case when untarring a rootfs for a container.
> 
> I guess my practical question is don't we want to reserve a range of
> subuid's for root owned containers as well.  Just so that we know
> someone is using those uids and we don't get them allocated to another
> purpose?
> 
> Or am I missing something here?

That's a not unreasonable argument, but two counters:  1. "someone"
using the uids used by root would mean root assigned those uids to
someone.  That's a misconfiguration, in other words this is keeping
root from shooting himself in the foot.  It's not in the kernel though,
so maybe it's ok.  2.  I like to think that any time something fails
as my user, if it fails due to permission reasons, then if I run it
as root it should work.  That's useful for debugging, and just seems
coherent with how most things work.

-serge