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

Steve Grubb
2011-09-07T20:02:41Z

On Wednesday, September 07, 2011 03:27:37 PM Ted Ts'o wrote:
> On Wed, Sep 07, 2011 at 02:26:35PM -0400, Jarod Wilson wrote:
> > 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.
>
> Yeah, but there are userspace programs that depend on urandom not
> blocking... so your proposed change would break them.

The only time this kicks in is when a system is under attack. If you have set this and
the system is running as normal, you will never notice it even there. Almost all uses
of urandom grab 4 bytes and seed openssl or libgcrypt or nss. It then uses those
libraries. There are the odd cases where something uses urandom to generate a key or
otherwise grab a chunk of bytes, but these are still small reads in the scheme of
things. Can you think of any legitimate use of urandom that grabs 100K or 1M from
urandom? Even those numbers still won't hit the sysctl on a normally function system.

When a system is underattack, do you really want to be using a PRNG for anything like
seeding openssl? Because a PRNG is what urandom degrades into when its attacked. If
enough bytes are read that an attacker can guess the internal state of the RNG, do you
really want it seeding a openssh session? At that point you really need it to stop
momentarily until it gets fresh entropy so the internal state is unknown. That's what
this is really about.

-Steve