linux-kernel - Re: [PATCH 1/4] Routine for generating an safe ID for kernel pointer

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Date:	Tue, 15 Nov 2011 16:13:40 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Helsley <matthltc@...ibm.com>
Subject: Re: [PATCH 1/4] Routine for generating an safe ID for kernel pointer

Le mardi 15 novembre 2011 à 15:36 +0400, Pavel Emelyanov a écrit :
> The routine XORs the given pointer with a random value thus producing
> an ID (32 or 64 bit, depending on the arch) which can be shown even to
> unprivileged user space processes without risking of leaking kernel
> information.
> 
> It implies that it gets called when the random pool is ready for providing
> a random long value.
> 
> Signed-off-by: Pavel Emelyanov <xemul@...allels.com>
> 
> ---
>  include/linux/gen_object_ids.h |   12 ++++++++++++
>  mm/Kconfig                     |    7 +++++++
>  mm/Makefile                    |    1 +
>  mm/gen_object_ids.c            |   21 +++++++++++++++++++++
>  4 files changed, 41 insertions(+), 0 deletions(-)
>  create mode 100644 include/linux/gen_object_ids.h
>  create mode 100644 mm/gen_object_ids.c
> 
> diff --git a/include/linux/gen_object_ids.h b/include/linux/gen_object_ids.h
> new file mode 100644
> index 0000000..17981ae
> --- /dev/null
> +++ b/include/linux/gen_object_ids.h
> @@ -0,0 +1,12 @@
> +#ifndef __GEN_OBJECT_IDS_H__
> +#define __GEN_OBJECT_IDS_H__
> +
> +#ifdef CONFIG_GENERIC_OBJECT_IDS
> +unsigned long gen_object_id(void *ptr);
> +#else
> +static inline unsigned long gen_object_id(void *ptr)
> +{
> +	return 0;
> +}
> +#endif
> +#endif
> diff --git a/mm/Kconfig b/mm/Kconfig
> index f2f1ca1..1480cbf 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -370,3 +370,10 @@ config CLEANCACHE
>  	  in a negligible performance hit.
>  
>  	  If unsure, say Y to enable cleancache
> +
> +config GENERIC_OBJECT_IDS
> +	bool "Enable generic object ids infrastructure"
> +	default n
> +	help
> +	  Turn on the (quite simple) funtionality that can generate IDs for
> +	  kernel objects which is safe to export to the userspace.
> diff --git a/mm/Makefile b/mm/Makefile
> index 836e416..155797a 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -50,3 +50,4 @@ obj-$(CONFIG_HWPOISON_INJECT) += hwpoison-inject.o
>  obj-$(CONFIG_DEBUG_KMEMLEAK) += kmemleak.o
>  obj-$(CONFIG_DEBUG_KMEMLEAK_TEST) += kmemleak-test.o
>  obj-$(CONFIG_CLEANCACHE) += cleancache.o
> +obj-$(CONFIG_GENERIC_OBJECT_IDS) += gen_object_ids.o
> diff --git a/mm/gen_object_ids.c b/mm/gen_object_ids.c
> new file mode 100644
> index 0000000..a75119b
> --- /dev/null
> +++ b/mm/gen_object_ids.c
> @@ -0,0 +1,21 @@
> +#include <linux/gen_object_ids.h>
> +#include <linux/spinlock.h>
> +#include <linux/random.h>
> +
> +static unsigned long ptr_poison __read_mostly;
> +static DEFINE_SPINLOCK(ptr_poison_lock);
> +
> +unsigned long gen_object_id(void *ptr)
> +{
> +	if (!ptr)
> +		return 0;
> +
> +	if (unlikely(!ptr_poison)) {
> +		spin_lock(&ptr_poison_lock);
> +		if (!ptr_poison)
> +			get_random_bytes(&ptr_poison, sizeof(ptr_poison));
> +		spin_unlock(&ptr_poison_lock);
> +	}
> +
> +	return ((unsigned long)ptr) ^ ptr_poison;
> +}

It can not be called from irq context then...

I suggest using following code instead (no lock needed)

if (!ptr_poison) {
	unsigned long rnd;

	do {
		get_random_bytes(&rnd, sizeof(rnd));
	} while (rnd == 0);
	cmpxchg(&ptr_poison, 0, rnd);
}
return ((unsigned long)ptr) ^ ptr_poison;



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