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

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Date:	Wed, 15 Feb 2012 20:27:52 +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 05:06:52PM +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.

> 
> > +static __init int kcmp_cookies_init(void)
> > +{
> > +       int i;
> > +
> > +       get_random_bytes(cookies, sizeof(cookies));
> > +
> > +       for (i = 0; i < KCMP_TYPES; i++)
> > +               cookies[i][1] |= (~(~0UL >>  1) | 1);
> 
> I am puzzled, help ;) this is equal to
> 
> 		cookies[i][1] |= -LONG_MAX;
> or
> 		cookies[i][1] |= (LONG_MIN | 1);
> 
> for what? why do we want to set these 2 bits (MSB and LSB) ?

Letme quote hpa@ here :)

 | This code is wrong.  You will have a zero cookie, legitimately, once in
 | 2^32 or 2^64 attempts, depending on the bitness.
 |
 | The other thing is that for the multiplicative cookie you should OR in
 | the value (~(~0UL >> 1) | 1) in order to make sure that the value is (a)
 | large and (b) odd.

	Cyrill
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