linux-kernel - Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always

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Date:	Thu, 12 Jan 2012 18:09:38 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Will Drewry <wad@...omium.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	oleg@...hat.com, ak@...ux.intel.com, eric.dumazet@...il.com,
	gregkh@...e.de, dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always_unprivileged" patch

On Thu, Jan 12, 2012 at 5:41 PM, Andrew Lutomirski <luto@....edu> wrote:
> On Thu, Jan 12, 2012 at 5:37 PM, Will Drewry <wad@...omium.org> wrote:
>> On Thu, Jan 12, 2012 at 7:11 PM, Andrew Lutomirski <luto@....edu> wrote:
>>> (Also, preventing dropping of privileges will probably make a patch
>>> more complicted -- I'll have to find and update all the places that
>>> allow dropping privileges.)
>>
>> An alternative approach might be that the restricted bit drops all
>> privileges that allows privilege changes in either direction.  E.g.,
>> - set restricted bit
>> -- adds a check anywhere MNT_NOSUID is
>> -- sets securebit to SECURE_NOROOT|..LOCKED
>> -- drops CAP_SETUID, CAP_DAC_OVERRIDE, ...
>> -- set the caps bounding set to the minimum the restricted bit allows
>>
>> That may deviate from the intent (by re-using caps), but it could keep some
>> of the privilege transition checking code the same.
>
> I'm not sure it'll be much of a simplification.  The entire patch is
> 45 lines right now :)  I'll test it and send it out.
>
> FWIW, though, it breaks apparmor (intentionally).  Can any of you
> either explain what *should* happen or (better) volunteer to fix it?
> It should be about three lines of code for someone who understands
> what's going on.  I don't have an apparmor system, so I can't really
> test it.

I'm happy to take a look at the AppArmor breakage. What's happening
with it at the moment?

-- 
Kees Cook
ChromeOS Security
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