linux-kernel - Re: [RFC PATCH] char: random: stir the output pools differently when the random

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Date:	Fri, 10 Jan 2014 13:15:34 +0100
From:	Stephan Mueller <stephan.mueller@...ec.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	Rafael Aquini <aquini@...hat.com>, Theodore Ts'o <tytso@....edu>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [RFC PATCH] char: random: stir the output pools differently when the random_write lenght allows splitting the seed

Am Freitag, 10. Januar 2014, 12:37:26 schrieb Clemens Ladisch:

Hi Clemens,

>Stephan Mueller wrote:
>> Am Freitag, 10. Januar 2014, 09:13:57 schrieb Clemens Ladisch:
>>> Rafael Aquini wrote:
>>>> This patch introduces changes to the random_write method so it can
>>>> split the given seed and completely stir the output pools with
>>>> different halves of it, when seed lenght allows us doing so.
>>>> 
>>>> -	ret = write_pool(&blocking_pool, buffer, count);
>>>> +	ret = write_pool(pool1, buffer, count1);
>>>> 
>>>>  	if (ret)
>>>>  	
>>>>  		return ret;
>>>> 
>>>> -	ret = write_pool(&nonblocking_pool, buffer, count);
>>>> +	ret = write_pool(pool2, buffer + offset, count2);
>>> 
>>> Doesn't this assume that both halves of the buffer contain some
>>> (uncredited) entropy?  In other words, wouldn't this result in worse
>>> randomness for pool2 if the second half of the buffer contains just
>>> zero padding?
>> 
>> [...]
>> Coming back to your concern: sure, the caller can pad any data
>> injected into /dev/?random with zeros.
>
>Assume that the userspace of an embedded device wants to do the same
>kind of initialization that a call to add_device_randomness() does, and
>that it has some data like "char serial_number[256]".  The padding
>wouldn't be done intentionally, it's just a property of the data (and
>it wouldn't have mattered before this patch).
>
>> But as writing to the character files is allowed to every user, this
>> per definition must not matter (e.g. an attacker may simply write
>> zeros or other known data into the character file). And the random.c
>> driver handles that case appropriately by not increasing the entropy
>> estimator when receiving data.
>
>The problem is not with the entropy estimate.
>
>> All the patch tries to achieve is to ensure that both pools are not
>> always mixed with the same values.
>
>Before this patch, both pools got mixed with the same values.  After
>this patch, both pools indeed get mixed with different values, but now
>one pool gets mixed with a known value if one half of the buffer
>happens to be known.

Do you imply in your example above that the serial number is unknown? 
Anything that unprivileged user space tries to inject into /dev/?random 
should be considered data with known value.

Ciao
Stephan
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