linux-kernel - Re: [patch 1/4] Add routine for generating an ID for kernel pointer

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Date:	Wed, 28 Dec 2011 08:06:55 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [patch 1/4] Add routine for generating an ID for kernel pointer

Hello, Cyrill.

Just my 2 cents.

On Fri, Dec 23, 2011 at 04:47:42PM +0400, Cyrill Gorcunov wrote:
> +unsigned long gen_obj_id(void *ptr, int type)
> +{
> +	if (!capable(CAP_SYS_ADMIN) || !ptr)
> +		return 0;
> +
> +	BUG_ON(type >= GEN_OBJ_ID_TYPES);
> +
> +	/*
> +	 * Note the simple XOR is used here not in a sake
> +	 * of security by any means, but rather to break
> +	 * an "impression" that such IDs means something
> +	 * other than a number which can be used for comparison
> +	 * with another number generated by this helper only.
> +	 */
> +	return ((unsigned long)ptr) ^ gen_obj_cookie[type];
> +}

To me, XOR & CAP_SYS_ADMIN combination doesn't make much sense.  With
CAP_SYS_ADMIN, there's no reason for XOR - we can just export
pointers.  If we plan on removing CAP_SYS_ADMIN restriction down the
road, XOR doesn't help much.  It's too weak.  The XOR is unnecessary
with CAP_SYS_ADMIN and useless without it.  It seems pointless to me.
If we're going down this route, I think doing cryptographically safe
hash would be much better.

Thanks.

-- 
tejun
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