linux-kernel - Re: LSM conversion to static interface

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Date:	Tue, 23 Oct 2007 10:20:05 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	Giacomo Catenazzi <cate@...ian.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andreas Gruenbacher <agruen@...e.de>,
	Thomas Fricaccia <thomas_fricacci@...oo.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	James Morris <jmorris@...ei.org>
Subject: Re: LSM conversion to static interface

Quoting Jan Engelhardt (jengelh@...putergmbh.de):
> 
> On Oct 23 2007 07:44, Giacomo Catenazzi wrote:
> >
> >> I do have a pseudo LSM called "multiadm" at 
> >> http://freshmeat.net/p/multiadm/ , quoting:
> >
> >> Policy is dead simple since it is based on UIDs. The UID ranges can be 
> >> set on module load time or during runtime (sysfs params). This LSM is 
> >> basically grants extra rights unlike most other LSMs[1], which is why 
> >> modprobe makes much more sense here. (It also does not have to do any 
> >> security labelling that would require it to be loaded at boot time 
> >> already.)
> >
> >But his is against LSM design (and first agreements about LSM):
> >LSM can deny rights, but it should not give extra permissions
> >or bypass standard unix permissions.
> 
> It is just not feasible to add ACLs to all million files in /home,
> also because ACLs are limited to around 25 entries.
> And it is obvious I do not want <prof> to have UID 0, because
> then you cannot distinguish who created what file.
> So the requirement to the task is to have unique UIDs.
> The next logical step would be to give capabilities to those UIDs.
> 
> *Is that wrong*? Who says that only UID 0 is allowed to have
> all 31 capability bits turned on, and that all non-UID 0 users
> need to have all 31 capability bits turned off?
> 
> So, we give caps to the subadmins (which is IMHO a natural task),
> and then, as per LSM design (wonder where that is written) deny
> some of the rights that the capabilities raised for subadmins grant,
> because that is obviously too much.

Once the per-process capability bounding set is accepted
(http://lkml.org/lkml/2007/10/3/315) you will be able to do something
like:

	1. Create user 'jdoe' with uid 0
	2. write a pam module which, when jdoe logs in, takes
	   CAP_NET_ADMIN out of his capability bounding set
	3. Now jdoe can log in with the kind of capabilities subset
	   you describe.

It's not a perfect solution, since it doesn't allow jdoe any way at all
to directly execute a file with more caps (setuid and file capabilities
are subject to the capbound).  So there is certainly still a place for
multiadm.

-serge
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