Re: [uml-devel] [PATCH 3/9] UML - "Hardware" random number generator

From: Rob Landley
Date: Thu Mar 10 2005 - 22:28:07 EST

On Wednesday 09 March 2005 09:15 pm, Jeff Dike wrote:
> This implements a hardware random number generator for UML which attaches
> itself to the host's /dev/random.

Direct use of /dev/random always makes me nervous. I've had a recurring
problem with /dev/random blocking, and generally configure as much as
possible to use /dev/urandom instead. It's really easy for a normal user to
drain the /dev/random entropy pool on a server (at least one that doesn't
have a sound card you can tell it to read white noise from). cat /dev/random
> /dev/null

I like /dev/urandom because it'll feed you as much entropy as it's got, but
won't block, and will presumably round-robin insert real entropy in the
streams that multiple users get from /dev/urandom. (I realize this may not
be the best place to get gpg keys from.)

I'm just thinking about those UML hosting farms, with several UML instances
per machine, on machines which haven't got a keyboard attached constantly
feeding entropy into the pool. If just ONE of them is serving ssl
connections from its own /dev/urandom, that would drain the /dev/random
entropy pool on the host machine almost immediately...

Admittedly if UML used /dev/urandom instead of /dev/random, it wouldn't know
how much "real" randomness it was getting and how much synthetic randomness,
but this makes predicting the numbers it's producing easier how?

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