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

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Message-ID: <87r4ms5wpm.fsf@xmission.com>
Date:	Fri, 14 Dec 2012 10:12:53 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	linux-security-module@...r.kernel.org
Subject: Re: [RFC][PATCH] Fix cap_capable to only allow owners in the parent user namespace to have caps.

"Serge E. Hallyn" <serge@...lyn.com> writes:

> Quoting Eric W. Biederman (ebiederm@...ssion.com):
>> "Serge E. Hallyn" <serge@...lyn.com> writes:
>> 
>> > Quoting Eric W. Biederman (ebiederm@...ssion.com):
>> >> "Serge E. Hallyn" <serge@...lyn.com> writes:
>> >> 
>> >> > Quoting Eric W. Biederman (ebiederm@...ssion.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]
>

Yes.  You have to nest uids.

>> > 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.

Right.  It does require having the uid of the owner of ns1 or ns2 in
ns3.  So you have to explicitly allow it.

What I don't see is any point in allowing something like that.


After taking a second look I just realized that this is completely
unexploitable with the code that is currently merged.  As creating
a grand child user namespace is competelely impossible.  Creating
a user namespace is requires capable(CAP_SYS_ADMIN) which is never
present in anything but the initial user namespace.


That said I think the current semantics of cap_capable are completely
fatal to reasoning about user namespaces.

A child user namespace having capabilities against processes in it's
parent seems totally bizarre and pretty dangerous from a capabilities
standpoint.

That said Serge I think I have lost track of the point of your question.

Eric
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