Re: [PATCH v5 0/7] /dev/random - a new approach

Theodore Ts'o
2016-06-21T05:14:50Z

On Mon, Jun 20, 2016 at 09:00:49PM +0200, Stephan Mueller wrote:
>
> The time stamp maintenance is the exact cause for the correlation: one HID
> event triggers:
>
> - add_interrupt_randomness which takes high-res time stamp, Jiffies and some
> pointers
>
> - add_input_randomness which takes high-res time stamp, Jiffies and HID event
> value
>
> The same applies to disk events. My suggestion is to get rid of the double
> counting of time stamps for one event.
>
> And I guess I do not need to stress that correlation of data that is supposed
> to be entropic is not good :-)

What is your concern, specifically? If it is in the entropy
accounting, there is more entropy in HID event interrupts, so I don't
think adding the extra 1/64th bit of entropy is going to be problematic.

If it is that there are two timestamps that are closely correleated
being added into the pool, the add_interrupt_randomness() path is
going to mix that timestamp with the interrupt timings from 63 other
interrupts before it is mixed into the input pool, while the
add_input_randomness() mixes it directly into the pool. So if you
think there is a way this could be leveraged into attack, please give
specifics --- but I think we're on pretty solid ground here.

Cheers,

- Ted