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

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Date:	Wed, 11 Apr 2012 01:08:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"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>,
	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 04/11, Cyrill Gorcunov wrote:
>
> --- linux-2.6.git.orig/kernel/kcmp.c
> +++ linux-2.6.git/kernel/kcmp.c
> @@ -17,6 +17,17 @@
>   * reasons, still the comparison results should be suitable for
>   * sorting. Thus, we obfuscate kernel pointers values and compare
>   * the production instead.
> + *
> + * The obfuscation is done in two steps. First -- we use xor on
> + * kernel pointer with random value, which puts pointer into
> + * a new position in reordered space. Second -- we multiply
> + * the xor production with big odd random number to permute
> + * bits even more (the oddity is important here, it allow
> + * us to have meaningful production even if multiplicants
> + * are big numbers).
> + *
> + * Note also the obfuscation itself is invisible to user-space
> + * and if needed it can be changed to any suitable scheme.
>   */
>  static unsigned long cookies[KCMP_TYPES][2] __read_mostly;

OK, since this is discussed again...

Can this comment can also explain why do we obfuscate the pointers
by type? I mean, I don't really understand why the one-dimensional
cookies[2] is "not enough" from security pov.

Oleg.

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