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    SubjectRe: [PATCH 2/2] lguest: virtio-rng support
    Date
    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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