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

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Message-ID: <482F0053.2000509@zytor.com>
Date:	Sat, 17 May 2008 08:57:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Johannes Berg <johannes@...solutions.net>,
	virtualization@...ts.linux-foundation.org,
	Jeff Garzik <jeff@...zik.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Theodore Tso <tytso@....edu>, Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH 2/2] lguest: virtio-rng support

Rusty Russell wrote:
>> Uhm, no.  It's not.  Unless the host provides actual entropy
>> information, you have a security hole.
> 
> Huh?  We just can't assume it adds entropy.  AFAICT rngd -H0 is what we want 
> here.

We can, if it comes from /dev/random.

>>> 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?
>> 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.
> 
> Seeding is good, but unlikely to be done properly for first boot of some 
> standard virtualized container.  In practice, feeding /dev/urandom from the 
> host will make /dev/urandom harder to predict in the guest.

Only up to a point.

>> The reason to provide anything at all from the host 
>> is to give it "golden" entropy bits.
> 
> But you did not address the DoS question: can we ignore it?  Or are we trading 
> off a DoS in the host against a potential security weakness in the guest?
> 
> If so, how do we resolve it?

I don't think you have a DoS situation at all.  The worst thing is that 
you don't have any entropy available at all, at which point /dev/urandom 
is as insecure as it ever is.

	-hpa

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