Re: [RFC PATCH] char: random: stir the output pools differently whenthe random_write lenght allows splitting the seed

[Clemens Ladisch]

Stephan Mueller wrote:
> Am Freitag, 10. Januar 2014, 12:37:26 schrieb Clemens Ladisch:
>> 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.

Like the kernel's add_device_randomness() function, this example assumes
that there is no persistent storage with a saved seed (or that it isn't
yet available), and that mixing a device-specific value at least
prevents multiple device instances from generating identical random
numbers.

This indeed helps only against attackers that do not know that serial
number.

If the data written by unprivileged users to /dev/?random were
considered known to *all* attackers, then it wouldn't make sense to
allow such writes at all.


Regards,
Clemens
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