Re: [PATCH] random: add blocking facility to urandom

[Jarod Wilson <jarod@redhat.com>]

Sasha Levin wrote:
> On Wed, 2011-09-07 at 13:38 -0400, Jarod Wilson wrote:
>> Certain security-related certifications and their respective review
>> bodies have said that they find use of /dev/urandom for certain
>> functions, such as setting up ssh connections, is acceptable, but if and
>> only if /dev/urandom can block after a certain threshold of bytes have
>> been read from it with the entropy pool exhausted. Initially, we were
>> investigating increasing entropy pool contributions, so that we could
>> simply use /dev/random, but since that hasn't (yet) panned out, and
>> upwards of five minutes to establsh an ssh connection using an
>> entropy-starved /dev/random is unacceptable, we started looking at the
>> blocking urandom approach.
>
> Can't you accomplish this in userspace by trying to read as much as you
> can out of /dev/random without blocking, then reading out
> of /dev/urandom the minimum between allowed threshold and remaining
> bytes, and then blocking on /dev/random?
>
> For example, lets say you need 100 bytes of randomness, and your
> threshold is 30 bytes. You try reading out of /dev/random and get 50
> bytes, at that point you'll read another 30 (=threshold) bytes
> out /dev/urandom and then you'll need to block on /dev/random until you
> get the remaining 20 bytes.

We're looking for a generic solution here that doesn't require 
re-educating every single piece of userspace. And anything done in 
userspace is going to be full of possible holes -- there needs to be 
something in place that actually *enforces* the policy, and centralized 
accounting/tracking, lest you wind up with multiple processes racing to 
grab the entropy.

-- 
Jarod Wilson
jarod@redhat.com