linux-kernel - Re: [PATCH RFC] Introduce new security.nscapability xattr

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Date:	Mon, 29 Feb 2016 15:38:20 -0600
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Serge E. Hallyn" <serge.hallyn@...ntu.com>
Cc:	Andy Lutomirski <luto@...capital.net>, Jann Horn <jann@...jh.net>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morgan <morgan@...nel.org>,
	LXC development mailing-list 
	<lxc-devel@...ts.linuxcontainers.org>,
	Richard Weinberger <richard@....at>,
	LSM <linux-security-module@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Christian Brauner <christian.brauner@...lbox.org>
Subject: Re: [PATCH RFC] Introduce new security.nscapability xattr

On Fri, Jan 29, 2016 at 01:31:51AM -0600, Serge E. Hallyn wrote:
> On Wed, Jan 27, 2016 at 04:36:02PM -0800, Andy Lutomirski wrote:
> > On Wed, Jan 27, 2016 at 9:22 AM, Jann Horn <jann@...jh.net> wrote:
> > > I think it sounds good from a security perspective.
> > 
> > I'm a bit late to the game, but I have a question: why should this be
> > keyed to the *root* uid of the namespace in particular?  Certainly if
> > user foo trusts the cap bits on some file, then user foo might trust
> > those caps to be exerted over any namespace that user foo owns, since
> > user foo owns the namespace.
> 
> ...  Tying it to a kuid which represents the userns->owner of any
> namespace in which the capability will be honored might be fine
> with me.  Is that what you mean?  So if uid 1000 creates a userns
> mapping uids 100000-200000, and 100000 in that container puts X=pe
> on /bin/foo, uid 101000 in that container runs /bin/foo with privilege
> X.  Uid 101000 in someone else's container does not.
> 
> Although, if I create two containers and provide them different
> uidmaps, it may well be because I want them segragated and want
> to minimize the changes of one container breaking out into the
> other.  This risks breaking that.

Thinking differently now...  I really want it to "just work" to tar
and untar these.  So I'm thinking of simply using the file owner
as the uid.  So to write a security.ns_capability xattr, you must
be uid 0 in the inode's namespace, the file must be owned by uid 0,
and the capabilities in the xattr will be honored for any namespace
where in that uid_t 0 is root.

Does that sound overly restrictive?  I expect file capabilities to
be used on files owned by root but not setuid-root, so I think it
is ok.

-serge