[Xen-devel] Re: [RFC PATCH 34/35] Add the Xen virtual network device driver.

[Stephen Hemminger <shemminger@xxxxxxxx>]

On Thu, 11 May 2006 11:47:52 +0200
Andi Kleen <ak@xxxxxxx> wrote:

> On Thursday 11 May 2006 09:49, Keir Fraser wrote:
> > On 11 May 2006, at 01:33, Herbert Xu wrote:
> > >> But if sampling virtual events for randomness is really unsafe (is it
> > >> really?) then native guests in Xen would also get bad random numbers
> > >> and this would need to be somehow addressed.
> > >
> > > Good point.  I wonder what VMWare does in this situation.
> >
> > Well, there's not much they can do except maybe jitter interrupt
> > delivery. I doubt they do that though.
> >
> > The original complaint in our case was that we take entropy from
> > interrupts caused by other local VMs, as well as external sources.
> > There was a feeling that the former was more predictable and could form
> > the basis of an attack. I have to say I'm unconvinced: I don't really
> > see that it's significantly easier to inject precisely-timed interrupts
> > into a local VM. Certainly not to better than +/- a few microseconds.
> > As long as you add cycle-counter info to the entropy pool, the least
> > significant bits of that will always be noise.
> 
> I think I agree - e.g. i would expect the virtual interrupts to have
> enough jitter too. Maybe it would be good if someone could
> run a few statistics on the resulting numbers?
> 
> Ok the randomness added doesn't consist only of the least significant
> bits. Currently it adds jiffies+full 32bit cycle count.  I guess if it was
> a real problem the code could be changed to leave out the jiffies and 
> only add maybe a 8 bit word from the low bits. But that would only
> help for the para case because the algorithm for native guests
> cannot be changed.
> 
> >   2. An entropy front/back is tricky -- how do we decide how much
> > entropy to pull from domain0? How much should domain0 be prepared to
> > give other domains? How easy is it to DoS domain0 by draining its
> > entropy pool? Yuk.
> 
> I claim (without having read any code) that in theory you need to have solved 
> that problem already in the vTPM @)
> 

The base question under all this is "how good does an entropy source have
to be?" and then "what guarantees do we make about the entropy inputs used
by /dev/random?".  If we can resolve those, then the virtual environment
answer should fall out.

This is a area where the security tin-foil hat types take over, and it
gets real hard to make "good enough" argument. People have built an expectation
that /dev/random has really strong entropy, good enough to generate long term
keys etc.

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