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

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Date:	Fri, 30 Dec 2011 18:51:37 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	Tejun Heo <tj@...nel.org>, 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>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [patch 1/4] Add routine for generating an ID for kernel pointer

(12/30/11 3:48 PM), Cyrill Gorcunov wrote:
> On Fri, Dec 30, 2011 at 03:31:32PM -0500, KOSAKI Motohiro wrote:
>> (12/30/11 2:36 AM), Cyrill Gorcunov wrote:
>>> On Fri, Dec 30, 2011 at 11:23:09AM +1100, Herbert Xu wrote:
>>>> On Thu, Dec 29, 2011 at 08:24:53PM +0400, Cyrill Gorcunov wrote:
>>>>>
>>>>> Probably I've had to crypto_alloc_hash earlier and simply keep a reference
>>>>> to algo but since I'm not sure if looking for modules in late-init-call
>>>>> is good idea.
>>>>
>>>> Right, the allocation needs to occur in a sleepable context.
>>>>
>>>> If you're just hashing something small and have no need for
>>>> hardware acceleration then lib/sha1.c is fine.
>>>>
>>>
>>> Hi, yeah, it's just one message block hashing so I've switched
>>> to lib/sha1.c. Herbert, I'm more interested in security analysis
>>> -- would the sha1(msg), where the 'msg' is the kernel pointer
>>> XOR'ed with random value and expanded to the 512 bits would be
>>> safe enough for export to unprivilege users?
>>
>> Even if now we don't know an attacking way of sha1 reverse hashing,
>> we may discover within 10 years. Many secure messages lost from
>> hardware speedup and new algorithm attack. so, nobody can say it's
>> abi safe.
>>
>
> Yes, I know. But there is a big difference between direct hash crack and
> indirect crack caused by limited space of data used for such hash. That's
> the reason why random cookie was used and xor production was expanded to
> the whole message block.
>
>> And, if you don't use perfect hash, you may have a hash collision
>> risk. What's happen if different pointer makes same ID?
>
> Well, strictly speaking this is pretty bad case for me. Of course this
> wont lead to catastrophic results for user-space application I think
> but definitely I would prefer to not have collisions here.
>
> Guys, this become more and more complex, finally I fear someone
> propose to do ideal hashing run-time ;) Maybe we can step back and
> live with root-only and plain pointers here? I'm not sure who else
> might need such facility except us, and if once there will be a candidate
> -- we could take a look on hashing again and provide safe hashes there. No?

But recently kernel security fashion are, we don't expose a kernel 
pointer at all even though the file is root only. I'm not sure how
much effective such fashion. but you seems run opposite way.

I doubt user land can implement good comparison way. Why you gave up 
Andrew's sys_are_these_files_the_same() idea?

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