linux-kernel - Re: LSM conversion to static interface

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Message-ID: <20071023153409.GA14215@vino.hallyn.com>
Date:	Tue, 23 Oct 2007 10:34:09 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	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 10:20, Serge E. Hallyn wrote:
> >
> >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
> 
> UID 0 is _not_ acceptable for me.

I'm aware.

> >	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 is not that easy.
> CAP_DAC_OVERRIDE is given to the subadmin to bypass the pre-security
> checks in kernel code, and then the detailed implementation of
> limitation is done inside multiadm.

You mean the read/write split?

> This is not just raising or lowering capabilities.

Nope, but it's related, and as I pointed out below it fits in pretty
nicely.

> >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.
> 
> A normal user can execute suid binaries today, and so can s/he with mtadm.
> I do not see where that will change - it does not need any caps atm.

And he will still be able to *run* the suid binary, but if cap_bound is
reduced he won't be able to use capabilities taken out of the bounding
set, multiadm loaded or not.

-serge
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