On Wed, Sep 19, 2012 at 8:29 PM, Pawel Jakub Dawidek <[email protected]> wrote: > On Wed, Sep 19, 2012 at 07:30:52PM +0100, Jonathan Anderson wrote: >> > If all the times are more or less equally probable in this range […] >> >> They're very unlikely to be equally probable. It would make sense to do some >> characterization of these times and their statistics: a highly non-uniform >> distribution would mean that we don't actually get many bits per attach. > > I have times for ~2000 device_attach() calls when loading sound card > driver on totally idle system. If someone could take those and analyse > the distribution that would be great. > >> > […] we have more >> > than 19 bits of entropy from this one call, but I reduced if to four >> > bits only, because there are devices that are much faster to attach. >> > >> >> Another reason for doing the above characterization is that, if a particular >> device_attach() really does provide 12 bits of uncertainty, it's a shame to >> drop eight of them on the floor. > > Rights. That's why I've prepared another patch: > > http://people.freebsd.org/~pjd/patches/harvest_device_attach.2.patch > > which effectively discards top ten bits, which means we expect 0.1% of > the attach time to be unpredictable (the attach time in most cases vary > by few percent, not sure yet how much of this variation is really > unpredictable).
This is the wrong thing to do! There's no reason to discard bits on input (modulo the device throwing away inputs, that is) - just reduce your entropy estimate. "Extra" bits do no harm. _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-security To unsubscribe, send any mail to "[email protected]"
