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

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Date:	Tue, 27 Dec 2011 15:33:04 -0800
From:	Andrew Morton <akpm@...ux-foundation.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>, 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>
Subject: Re: [patch 1/4] Add routine for generating an ID for kernel pointer

On Fri, 23 Dec 2011 16:47:42 +0400
Cyrill Gorcunov <gorcunov@...nvz.org> wrote:

> The routine XORs the given pointer with a random value
> producing an ID (32 or 64 bit, depending on the arch).
> 
> Since it's a valuable information -- only CAP_SYS_ADMIN
> is allowed to obtain it.
> 
>  - Tejun worried about the single poison value was a weak side -
>    leaking one makes all the IDs vulnerable. To address this
>    several poison values - one per object type - are introduced.
>    They are stored in a plain array.
>  - Pekka proposed to initialized poison values in the late_initcall callback
>  - ... and move the code to mm/util.c
>
> ...
>

The code in general looks simple and reasonable to me.  I'm too much of
a security weenie to pass judgement on the security aspects.

>
> ...
>
> --- linux-2.6.git.orig/mm/Kconfig
> +++ linux-2.6.git/mm/Kconfig
> @@ -373,3 +373,19 @@ config CLEANCACHE
>  	  in a negligible performance hit.
>  
>  	  If unsure, say Y to enable cleancache
> +
> +config GENERIC_OBJECT_ID
> +	bool "Enable generic object ID infrastructure"
> +	depends on CHECKPOINT_RESTORE

Is c/r useless without GENERIC_OBJECT_ID?  If so, perhaps a `select'
would be good here.

> +	default n
> +	help
> +	  Turn on the functionality that can generate IDs for kernel
> +	  objects, which are exported to userspace via /proc filesystem.
> +
> +	  It is useful if you need to examinate kernel objects and test
> +	  if they are shared between several tasks. These IDs should never
> +	  be used for anything but the "sameness" test. Besides, the IDs are
> +	  dynamic and valid only while object is alive, once it get freed or
> +	  kernel is rebooted -- the IDs will be changed.
> +
> +	  If unsure, say N here.
> Index: linux-2.6.git/mm/Makefile
> ===================================================================
> --- linux-2.6.git.orig/mm/Makefile
> +++ linux-2.6.git/mm/Makefile
> @@ -51,3 +51,4 @@ obj-$(CONFIG_HWPOISON_INJECT) += hwpoiso
>  obj-$(CONFIG_DEBUG_KMEMLEAK) += kmemleak.o
>  obj-$(CONFIG_DEBUG_KMEMLEAK_TEST) += kmemleak-test.o
>  obj-$(CONFIG_CLEANCACHE) += cleancache.o
> +obj-$(CONFIG_GENERIC_OBJECT_ID) += gen_obj_id.o
> Index: linux-2.6.git/mm/gen_obj_id.c
> ===================================================================
> --- /dev/null
> +++ linux-2.6.git/mm/gen_obj_id.c
> @@ -0,0 +1,51 @@
> +#include <linux/kernel.h>
> +#include <linux/capability.h>
> +#include <linux/random.h>
> +#include <linux/module.h>
> +#include <linux/init.h>
> +#include <linux/gen_obj_id.h>

Formally, we need more includes than this.  cache.h for __read_mostly,
bug.h for BUG(), maybe others.  Forgetting bug.h used to be (and maybe
still is) a popular way of breaking the build for alpha.

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