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Re: [PATCH 2/2] lguest: virtio-rng support

From:  "H. Peter Anvin" <>
To:  Rusty Russell <>
Subject:  Re: [PATCH 2/2] lguest: virtio-rng support
Date:  Fri, 16 May 2008 21:50:31 -0700
Message-ID:  <>
Cc:  Theodore Tso <>, Herbert Xu <>, Jeff Garzik <>, LKML <>,, Christian Borntraeger <>, Matt Mackall <>, Johannes Berg <>
Archive-link:  Article

Rusty Russell wrote:
> On Friday 16 May 2008 20:49:41 Johannes Berg wrote:
>>> +
>>> +/* Our random number generator device reads from /dev/urandom into the Guest's
>>> + * input buffers.  The usual case is that the Guest doesn't want random numbers
>>> + * and so has no buffers although /dev/urandom is still readable, whereas
>>> + * console is the reverse. 
>> Is it really a good idea to use the hosts /dev/urandom to fill the
>> guests /dev/random?
> Technically it's up to rngd in the guest to decide whether to feed entropy
> or not (ie. /dev/urandom or /dev/random).

Uhm, no.  It's not.  Unless the host provides actual entropy 
information, you have a security hole.

> If we use /dev/random in the host, we risk a DoS.  But since /dev/random
> is 0666 on my system, perhaps noone actually cares?

/dev/random	= give me actual entropy, if you have some.
/dev/urandom	= give me what you have, regardless of quality.

There is no point in feeding the host /dev/urandom to the guest (except 
for seeding, which can be handled through other means); it will do its 
own mixing anyway.  The reason to provide anything at all from the host 
is to give it "golden" entropy bits.


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