linux-kernel - Re: [patch 1/2] c/r: prctl: Add ability to set new mm_struct::exe

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Date:	Tue, 20 Mar 2012 02:39:41 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>
Subject: Re: [patch 1/2] c/r: prctl: Add ability to set new
 mm_struct::exe_file

On Mon, Mar 19, 2012 at 03:15:07PM -0700, Andrew Morton wrote:
...
> > 
> > Also this action is one-shot only. For security reason
> > we don't allow to change the symlink several times.
> 
> What is this mysterious "security reason"?
> 

Oh, sorry I should have included Matt's comment here

	| Before this patch that state was rather ephemeral and almost entirely
	| under the control of the kernel. The only way userspace could change it
	| was by unmapping the region(s) mapped during exec*(). At that point it
	| could not "lie" and insert some other symlink there and the admin would
	| be better able to determine what had happened.
	|
	| With this patch -- especially the multi-shot form -- the symlink will
	| be entirely under the control of (potentially untrusted) userspace code
	| and the admin is totally at the mercy of the userspace code. In
	| single-shot form programs could use the prctl() to ensure the symlink
	| could not be changed later -- the restart tool would be the only program
	| that would need to ensure that prctl() had not been used since the last
	| exec*().
...
> 
> It should be explained here also.  The comment is pretty useless - if
> we don't tell people what this "security reason" is, how can future
> developers be sure that they aren't violating it?
> 

Actually I liked multi-shot version more but Matt arguments convinced
me that one-short fashion is more "secure" in terms of overall kernel
state and potential transitions/changes of this /proc/pid/exe symlink.

At least with one-shot version the admin may be sure that the symlink
is never changed more than once, ever.

	Cyrill
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