linux-kernel - Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

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Date:	Wed, 15 Feb 2012 21:53:19 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Alexey Dobriyan <adobriyan@...il.com>, Valdis.Kletnieks@...edu,
	Michal Marek <mmarek@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

On Wed, Feb 15, 2012 at 05:22:22PM +0100, Oleg Nesterov wrote:
> 
> > So I don't understand how it's different from what
> > is provided in this patch. What I'm missing?
> 
> environ_read() does
> 
> 	mm = mm_access(task);
> 	if (mm)
> 		do_something(mm);
> 
> even if it races with, say, execve(setuid_app) we can't read the
> new ->mm.

Wait, I'm confused

	process 1 (reader)	process 2 ("task" itself)
	mm = mm_access(task);
				task changes own credentials
				so reader can't access on next
				read if it would try, but since
				access already granted... it
				continues do_something(mm)
	if (mm)
		do_something(mm);

So in the patch I tried the same, once access is granted it
belongs to a caller.

> 
> while your code (very roughly) does something like
> 
> 	mm = mm_access(task);
> 	if (mm)
> 		do_something(task->mm);
> 
> while it is quite possible that mm != task->mm.

Oleg, could you please explain me where it happens
that task->mm (I've got access to) will be changed
to some new -mm while I'm inspecting it.

If permission changed while the caller inside syscall,
it's the same situation as with mm_access above. No?

> 
> My only point is: this check is obviously racy, and thus it looks
> confusing. Whether this is fine or not, I do not know. Personally
> I see no reason for ptrace_may_access(), but I am not security
> expert.

The idea was -- non-privilege caller should not have access
to privileged tasks.

	Cyrill
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