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    SubjectRe: [RFC PATCH 34/35] Add the Xen virtual network device driver.
    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 @)

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