Date: Wed, 15 Feb 2012 20:27:52 +0400
From: Cyrill Gorcunov <gorcunov@openvz.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Pavel Emelyanov <xemul@parallels.com>,
        Andrey Vagin <avagin@openvz.org>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Glauber Costa <glommer@parallels.com>,
        Andi Kleen <andi@firstfloor.org>, Tejun Heo <tj@kernel.org>,
        Matt Helsley <matthltc@us.ibm.com>, Pekka Enberg <penberg@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Vasiliy Kulikov <segoon@openwall.com>,
        Alexey Dobriyan <adobriyan@gmail.com>, Valdis.Kletnieks@vt.edu,
        Michal Marek <mmarek@suse.cz>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree
Message-ID: <20120215162752.GF4533@moon>
References: <20120215143606.GA14037@redhat.com>
 <20120215160652.GA17680@redhat.com>
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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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