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Date: Tue, 24 Jan 2012 00:03:47 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Pavel Emelyanov <xemul@parallels.com>,
        Serge Hallyn <serge.hallyn@canonical.com>,
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        "Eric W. Biederman" <ebiederm@xmission.com>,
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        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
        Glauber Costa <glommer@parallels.com>,
        Andi Kleen <andi@firstfloor.org>, Matt Helsley <matthltc@us.ibm.com>,
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        Eric Dumazet <eric.dumazet@gmail.com>,
        Vasiliy Kulikov <segoon@openwall.com>, Valdis.Kletnieks@vt.edu
Subject: Re: [patch 2/4] [RFC] syscalls, x86: Add __NR_kcmp syscall v4
Message-ID: <20120123200347.GK1907@moon>
References: <20120123142036.025893883@openvz.org>
 <20120123142436.258631438@openvz.org>
 <4F1DAB99.4060008@zytor.com>
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On Mon, Jan 23, 2012 at 10:48:57AM -0800, H. Peter Anvin wrote:
> On 01/23/2012 06:20 AM, Cyrill Gorcunov wrote:
> > +
> > +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];
> > +}
> > +
> 
> Arf... when I said to use xor I meant instead of the add, not instead of
> the multiply, so:
> 
> 	return (v ^ cookies[type][0]) * cookies[type][1];
> 
> Otherwise you have absolutely no source of diffusion at all (symmetric
> cryptography is about combinations of diffusion -- spreading the content
> -- and confusion -- scrambling individual bits of content.)
> 
> +	for (i = 0; i < KCMP_TYPES; i++) {
> +		for (j = 0; j < 2; j++) {
> +			get_random_bytes(&cookies[i][j],
> +					 sizeof(cookies[i][j]));
> +			cookies[i][j] |= (~(~0UL >>  1) | 1);
> +		}
> +	}
> 
> Only cookies[1] -- being used as a multiplicative constant -- needs the OR.

OK. Thanks. I'll update.

	Cyrill