linux-kernel - Re: [PATCH v2 0/4] Checkpoint/Restore: Show in proc IDs of objects that can be shared between tasks

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Date:	Fri, 18 Nov 2011 20:09:12 -0500
From:	Kyle Moffett <kyle@...fetthome.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>
Subject: Re: [PATCH v2 0/4] Checkpoint/Restore: Show in proc IDs of objects
 that can be shared between tasks

On Fri, Nov 18, 2011 at 18:46, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Fri, Nov 18, 2011 at 05:10:37PM -0500, Kyle Moffett wrote:
>> The #1 rule of one-time pads is never use it for more than one thing,
>> and you use it here for every object in the system.
>
> The new version is using different poison for different types of
> objects.

Even still, if you use a one-time pad (IE: XOR with a random data
value) to obscure more than exactly 1 object total, ever, all of its
security properties are null and void.

>> If you actually want to be able to compare uniqueness without exposing
>> anything vulnerable to various kinds of guessing, you should generate
>> a random 64-bit value for each class of object and then use a proper
>> cryptographic hash function on it:
>>   crypto_hash(concat(object_ptr, random_value))
>>
>> Even given the best possible practical attacks against SHA1 or MD5
>> today both still provides more than enough security to prevent someone
>> from guessing "object_ptr" in less than an absurd amount of time.
>
> So, per-type poison + crypto hash, it is then.

Yes.  I haven't thought through whether or not you would ever care
about a system giving out the same number for two different kinds of
object.  The only possible vulnerability I can think of would be if
the kernel had a use-after-free bug... You could allocate and free a
bunch of the vulnerable objects and use this data-structure-ID system
to find an allocated data-structure of a different type which matches
up with one of the used-after-freed ones.  Then in theory you could
compromise something, I suppose.

Sort of an off-the-wall scenario, I will admit.

The per-type random value is certainly a safe bet and should have zero
actual impact on performance.  Good luck!

Cheers,
Kyle Moffett

-- 
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