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

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Message-ID: <20120409222443.GW1625@moon>
Date:	Tue, 10 Apr 2012 02:24:43 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"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>,
	linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

On Mon, Apr 09, 2012 at 03:10:27PM -0700, Andrew Morton wrote:
> Back on to kcmp.
> 
> On Wed, 15 Feb 2012 20:27:52 +0400
> Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> 
> > On Wed, Feb 15, 2012 at 05:06:52PM +0100, Oleg Nesterov wrote:
> > > Not a comment, but the question. I am just curious...
> > > 
> > > > +/*
> > > > + * We don't expose real in-memory order of objects for security
> > > > + * reasons, still the comparison results should be suitable for
> > > > + * sorting. Thus, we obfuscate kernel pointers values and compare
> > > > + * the production instead.
> > > > + */
> > > > +static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
> > > > +
> > > > +static long kptr_obfuscate(long v, int type)
> > > > +{
> > > > +       return (v ^ cookies[type][0]) * cookies[type][1];
> > > > +}
> > > 
> > > OK, but why do we need this per type? Just to add more obfuscation
> > > or there is another reason?
> > 
> > Just to add more obfuscation.
> 
> Having re-read most of the (enormous) email discussion on the kcmp()
> syscall patch, I'm thinking:
> 
> - Nobody seems to understand the obfuscation logic.  Jon sounded
>   confused, Oleg sounds confused and it's rather unclear what it does,
>   how it does it and why it does it.

The obfuscation logic was done with great help from hpa@. And the main
idea was to have ordered results after obfuscation. Per-type noise increase
randomization of results. So Andrew, I actually dont know what to add
here. We don't want to provide kernel order back to user-space in
naked manner.

> 
> - Lots of people have looked at the code and made comments and there
>   have been lots of changes.  But we presently have zero Acked-by's and
>   Reviewed-by's.
> 

I guess I can ask hpa@ and Eric for Reviewed-by or Acked-by tag?

> I guess this means that at present nobody is aware of any issues with
> the proposal, btu nobody is terribly excisted about it either?
> 

I would rather say not much people yet use it.

> So what do people think?  Any issues?  Any nacks?  Should I sneak it
> into Linus this week or do we need to go another round with it all?
> 
> I'd like to at least have a fighting chance of understnading what's
> going on with that obfuscation code.

	Cyrill
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