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

Neil Horman
2011-09-07T20:33:17Z

On Wed, Sep 07, 2011 at 04:02:24PM -0400, Steve Grubb wrote:
> 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
Theres no way you can guarantee that. A quick lsof on my system here shows 27
unique pids that are holding /dev/urandom open, and while they may all be small reads,
taken in aggregate, I can imagine that they could pull a significant amount of
entropy out of /dev/urandom.

> 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.
>
How can you be sure of that? This seems to make assumptions about both the rate
at which entropy is drained from /dev/urandom and the limit at which you will
start blocking, neither of which you can be sure of.

> When a system is underattack, do you really want to be using a PRNG for anything like
How can you be sure that this only happens when a system is under some sort of
attack. /dev/urandom is there for user space to use, and we can't make
assumptions as to how it will get drawn from. What if someone was running some
monte-carlo based test program? That could completely exhaust the entropy in
/dev/urandom and would be perfectly legitimate.

> 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.
I never really want my ssh session to be be seeded with non-random data. Of
course, in my mind thats an argument for making ssh use /dev/random rather than
/dev/urandom, but I'm willing to take the tradeoff in speed most of the time.

>
> -Steve