Messages in this thread | | | Subject | Re: mmotm 2008-12-01-19-41: early exception (page fault -- deref of 0x20) | From | Matt Mackall <> | Date | Wed, 03 Dec 2008 11:07:00 -0600 |
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On Tue, 2008-12-02 at 15:50 -0800, Arjan van de Ven wrote: > Matt Mackall wrote: > > > > If we oops or warn while picking a timesource, we'll have lots of fun? > > > > we really only need to mix in the tsc; ktime_get() is just an arch friendly way to get that > I supposed (wrongly). > > but yes we need to do a few things > 1) seed on demand with a platform time source
Currently we use jiffies + get_cycles(). That's going to have somewhere between, oh, 3 bits of entropy (very stable boot with only jiffies) and 25 bits of entropy (TSC with lots of waiting for hardware) at boot. Ideally, we'd have access to a wall clock of some sort as well. But that's also a fairly limited source - wall clocks are both low resolution and predictable/collision-prone.
> 2) have a way where arch init can just hand semi random data during the boot process to > increase the randomness (even if it doesn't count as entropy)
A simple wrapper around mix_pool_bytes probably fits the ticket.
But I don't think this will solve the general problem of 'large numbers of practically identical machines booting up with the same pre-init random number pools'. Beyond things like MAC addresses and serial numbers (predictable/observable but at least not collision-prone), we have no way to differentiate some boxes. We may need to forcibly generate some timing entropy. Perhaps something like this:
http://markmail.org/message/xwsbywr6ziil2qu2
(which is way too slow in its current form)
There's a related problem of systems with no way to store a seed across boots.
-- Mathematics is the supreme nostalgia of our time.
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