From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Paris <eparis@redhat.com>
Subject: Re: [PATCH] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from
 granting privs
Date: Fri, 13 Jan 2012 15:13:25 -0500
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Cc: Andy Lutomirski <luto@amacapital.net>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Oleg Nesterov <oleg@redhat.com>,
	Will Drewry <wad@chromium.org>, linux-kernel@vger.kernel.org,
	keescook@chromium.org, john.johansen@canonical.com,
	serge.hallyn@canonical.com, coreyb@linux.vnet.ibm.com,
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	luto@mit.edu, mingo@elte.hu, akpm@linux-foundation.org,
	khilman@ti.com, borislav.petkov@amd.com, amwang@redhat.com,
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	dhowells@redhat.com, daniel.lezcano@free.fr,
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	linux-security-module@vger.kernel.org, olofj@chromium.org,
	mhalcrow@google.com, dlaor@redhat.com, corbet@lwn.net
To: Linus Torvalds <torvalds@linux-foundation.org>
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On Fri, 2012-01-13 at 11:45 -0800, Linus Torvalds wrote:
> On Fri, Jan 13, 2012 at 11:39 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > Is the current exec_no_trans check enough for you?  With my patch,
> > selinux can already block the execve if it wants.
> 
> If this feature has "selinux can do xyz if it wants", it is broken.
> 
> The *whole* point is to get the f*^%ing crazy "security managers can
> do xyz" things away from it.
> 
> The flag - when set - should give a 100% guarantee that security
> context doesn't change, and an operation that would change it would
> error out.

That's what you would get today following the MNT_NOSUID example.
SELinux just has the additional property that the policy can either
error (and fail the exec) or allow no selinux transition to happen.  

> Not a "selinux can block it if it wants". None of that "wants" crap.
> None of the "you can configure security rules to do xyz" crap.
> 
> One simple rule: no security changes from the context that set the flag.
> 
> Any other rule will inevitably cause random gray areas where some
> random security manager does something stupid. We have enough of those
> already. No more.

So you can't drop capabilities(7)?  If you come in with permission you
can't get rid of it?  Ouch.

My thought on expanding the SELinux support beyond 'no
transition' (which I suggest we do today) would be that we might allow
SELinux transitions if we can show the the 'child' domain is a subset of
the 'parent' domain. Much the same as I imagine you can still drop
capabilities after setting this flag you might be able to drop SELinux
permissions, but that's something that would need a lot of thought and
that we don't have a good way to do today...