Re: [RFC][PATCH] Fix cap_capable to only allow owners in the parent user namespace to have caps.

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serge@hallyn.com> writes:
> 
> > Quoting Eric W. Biederman (ebiederm@xmission.com):
> >> "Serge E. Hallyn" <serge@hallyn.com> writes:
> >> 
> >> > Quoting Eric W. Biederman (ebiederm@xmission.com):
> >> >> 
> >> >> Andy Lutomirski pointed out that the current behavior of allowing the
> >> >> owner of a user namespace to have all caps when that owner is not in a
> >> >> parent user namespace is wrong.
> >> >
> >> > To make sure I understand right, the issue is when a uid is mapped
> >> > into multiple namespaces.
> >> 
> >> Yes.
> >> 
> >> i.e. uid 1000 in ns1 may own ns2, but uid 1000 in ns3 does not?
> >> 
> >> I am not certain of your example.
> >> 
> >> The simple case is:
> >> 
> >> init_user_ns:
> >>      child_user_ns1 (owned by uid == 0 [in all user namespaces])
> >>            child_user_ns2 (owned by uid == 0 [ in all user namespaces])
> >> 
> >> 
> >> root (uid == 0) in child_user_ns2 has all rights over anything in
> >> child_user_ns1.
> >
> > Well that is only if there was no mapping.  (since we're comparing
> > kuids, not uid_ts).  right?  If you didn't map uid 0 in child_user_ns2
> > to another id in the parent ns, you weren't all *that* serious about
> > isolating the ns.
> >
> > The case I was thinking is
> >
> >   init_user_ns:  [0-uidmax]
> >       child_user_ns1  [100000-199999]
> >       child_user_ns2  [100000-199999]
> >         child_user_ns3  [200000-299999]

Wait is my example above possible?  Or does child_user_ns3's range need
to be a subset of child_user_ns2's?

In which case it would be

       child_user_ns1  [100000-199999]
       child_user_ns2  [100000-199999]
         child_user_ns3  [120000-129999]

> > with unfortunate mappings  - ns1 and ns2 should have had nonoverlapping
> > ranges, but in any case now uid 1000 in ns1 can exert privilege over
> > ns3.  Again, uids comparisons will succeed for file access anyway, so
> > ns1 can 0wn ns2 and ns3 other ways.
> 
> Yes yours is the more realistic scenario.  Mine was simplified to be clear.
> 
> > Heck I'm starting to think the bug is a feature - surely given the
> > mappings above I meant for ns1 and ns2 to bleed privilege to each
> > other?
> 
> The serious problem is that privileges can bleed up. A user in 
> ns3 can wind up owning ns2 or ns1.  Which totally defeats the permission
> model.  You have CAP_DAC_OVERRIDE so you don't even need access to files
> you own, etc, etc.

Would that not require intervention from the init_user_ns?  In my
example above (let's add that ns2 is owned by kuid.uid=1000 in
init_user_ns), root in child_user_ns2 cannot map kuid.val=0 or
kuid.val=1000 into ns3 because 0 and 1000 are not in the range
100000-199999.  So there is no uid in child_user_ns3 which is able
to spoof uid=0 in child_user_ns1.

-serge