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

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Message-Id: <200805171628.03801.rusty@rustcorp.com.au>
Date:	Sat, 17 May 2008 16:28:03 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"H. Peter Anvin" <hpa@...or.com>
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

On Saturday 17 May 2008 14:50:31 H. Peter Anvin wrote:
> 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.

Huh?  We just can't assume it adds entropy.  AFAICT rngd -H0 is what we want 
here.

> > 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.

> 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?

Thanks,
Rusty.
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