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

[Tejun Heo]

Hello,

On Thu, Dec 29, 2011 at 06:24:38PM +0400, Cyrill Gorcunov wrote:
> Tejun, I've tried to use crypto engine here but it produced a warning
> about being used in non-sleepable context (which takes place when we
> read /proc/<pid>/fdinfo/* files). So I used lib/sha1.c instead. The
> final result is below, please review.

I don't know anything about cryptography and have no idea whether sha1
is good enough, so I can't really say much. :)

Which part triggered the context warning?  IIRC, crypto context
preparation and actual calculation can be done in separate steps.
Can't the calculation part be done from non-sleepable context?

cc'ing Herbert & David and quoting the whole message.

Thanks.

> The output as expected is sha1 hash but note I still placed root-only
> access here since I prefer security guys to confirm if such production
> is indeed safe to be exported for regular users.
> 
> | [root@localhost ~]# cat /proc/2/ns/net 
> | id:	ebfc829fc16a466c9c02b031ceab65c277040c02
> 
> 	Cyrill
> ---
> From: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Subject: Add routine for generating an ID for kernel pointer v2
> 
> The routine XORs the given pointer with a random value,
> expands the production to sha1 message block and compute
> the sha1 hash producing an ID.
> 
> Util confirmed that the production is a safe one to be
> exported for the unprivileged users CAP_SYS_ADMIN guird
> is used.
> 
> Because the routine might be used in non-sleepable context
> built-in realization of sha1 is chosen instead of crypto
> manager and its helpers.
> 
> v1:
>  - 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, but eventually it's moved
>    to an own file.
> 
> v2:
>  - Tejun suggested to use crypto engine instead of plain XOR and
>    provide hashes to unprivilege users as well.
>  - A number of fixes for Kconfig prompt from Valdis
>  - Andrew pointed more #incclude's are needed
> 
> Based-on-patch-from: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> CC: Glauber Costa <glommer@xxxxxxxxxxxxx>
> CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
> CC: Tejun Heo <tj@xxxxxxxxxx>
> CC: Matt Helsley <matthltc@xxxxxxxxxx>
> CC: Pekka Enberg <penberg@xxxxxxxxxx>
> CC: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> CC: Vasiliy Kulikov <segoon@xxxxxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> CC: Valdis.Kletnieks@xxxxxx
> ---
>  include/linux/gen_obj_id.h |   26 ++++++++++++++
>  mm/Kconfig                 |   18 ++++++++++
>  mm/Makefile                |    1 
>  mm/gen_obj_id.c            |   81 +++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 126 insertions(+)
> 
> Index: linux-2.6.git/include/linux/gen_obj_id.h
> ===================================================================
> --- /dev/null
> +++ linux-2.6.git/include/linux/gen_obj_id.h
> @@ -0,0 +1,26 @@
> +#ifndef _LINUX_GEN_OBJ_ID_H
> +#define _LINUX_GEN_OBJ_ID_H
> +
> +#include <linux/cryptohash.h>
> +#include <linux/err.h>
> +
> +#ifdef __KERNEL__
> +
> +enum {
> +	GEN_OBJ_ID_TYPES,
> +};
> +#define GEN_OBJ_ID_DIGEST_SIZE	(SHA_DIGEST_WORDS * sizeof(__u32))
> +#define GEN_OBJ_ID_BUF_SIZE	(GEN_OBJ_ID_DIGEST_SIZE * 2)
> +
> +#ifdef CONFIG_GENERIC_OBJECT_ID
> +extern int gen_obj_id(void *ptr, int type, char *dst, unsigned long size);
> +#else
> +static inline int gen_obj_id(void *ptr, int type, char *dst, unsigned long size)
> +{
> +	return -EINVAL;
> +}
> +#endif
> +
> +#endif /* __KERNEL__ */
> +#endif /* _LINUX_GEN_OBJ_ID_H */
> Index: linux-2.6.git/mm/Kconfig
> ===================================================================
> --- linux-2.6.git.orig/mm/Kconfig
> +++ linux-2.6.git/mm/Kconfig
> @@ -373,3 +373,21 @@ 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
> +	depends on CRYPTO_SHA1
> +	default n
> +	help
> +	  Turn on functionality that can generate IDs for kernel
> +	  objects, which are exported to the userspace via /proc
> +	  filesystem.
> +
> +	  It is useful if you need to examine kernel objects and test
> +	  if they are shared between several tasks. These IDs should never
> +	  be used for anything but the "sameness" test. 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 +1,81 @@
> +#include <linux/kernel.h>
> +#include <linux/capability.h>
> +#include <linux/cryptohash.h>
> +#include <linux/string.h>
> +#include <linux/random.h>
> +#include <linux/module.h>
> +#include <linux/init.h>
> +#include <linux/cache.h>
> +#include <linux/bug.h>
> +#include <linux/err.h>
> +
> +#include <linux/gen_obj_id.h>
> +
> +static unsigned long gen_obj_cookie[GEN_OBJ_ID_TYPES] __read_mostly;
> +
> +int gen_obj_id(void *ptr, int type, char *dst, unsigned long size)
> +{
> +	__u32 hash[SHA_DIGEST_WORDS];
> +	__u32 workspace[SHA_WORKSPACE_WORDS];
> +	__u8 extract[SHA_MESSAGE_BYTES];
> +	__u8 *t = (__u8 *)hash;
> +	unsigned long id;
> +	int i;
> +
> +	BUG_ON(type >= GEN_OBJ_ID_TYPES);
> +
> +	if (!capable(CAP_SYS_ADMIN) ||
> +	    size < GEN_OBJ_ID_BUF_SIZE) {
> +		memset(dst, '0', size);
> +		return -EINVAL;
> +	}
> +
> +	id = ((unsigned long)ptr) ^ gen_obj_cookie[type];
> +
> +	BUILD_BUG_ON(SHA_MESSAGE_BYTES % sizeof(long));
> +
> +	/*
> +	 * Expand the ID to the whole message block.
> +	 */
> +	for (i = 0; i < (SHA_MESSAGE_BYTES / sizeof(long)); i++)
> +		((long *)extract)[i] = id;
> +
> +	sha_init(hash);
> +	sha_transform(hash, extract, workspace);
> +
> +	snprintf(dst, size,
> +		 "%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x"
> +		 "%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
> +		 t[ 0], t[ 1], t[ 2], t[ 3], t[ 4],
> +		 t[ 5], t[ 6], t[ 7], t[ 8], t[ 9],
> +		 t[10], t[11], t[12], t[13], t[14],
> +		 t[15], t[16], t[17], t[18], t[19]);
> +
> +	return 0;
> +}
> +
> +static __init int gen_obj_cookie_init(void)
> +{
> +#if BITS_PER_LONG == 64
> +	const unsigned long emergency_cookie = 0xefcdab8967452301;
> +#else
> +	const unsigned long emergency_cookie = 0x98badcf9;
> +#endif
> +	int i;
> +
> +	for (i = 0; i < GEN_OBJ_ID_TYPES; i++) {
> +		get_random_bytes(&gen_obj_cookie[i],
> +				 sizeof(unsigned long));
> +		/*
> +		 * In 'impossible' case of random-bytes = 0
> +		 * we still would have non-zero value.
> +		 */
> +		gen_obj_cookie[i] =
> +			(gen_obj_cookie[i] & __PAGE_OFFSET) +
> +			(emergency_cookie & ~__PAGE_OFFSET);
> +	}
> +
> +	return 0;
> +}
> +
> +late_initcall(gen_obj_cookie_init);

-- 
tejun
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