On Tue, 2011-06-14 at 11:18 -0400, Jarod Wilson wrote: > Matt Mackall wrote: > > On Mon, 2011-06-13 at 18:06 -0400, Jarod Wilson wrote: > >> Many server systems are seriously lacking in sources of entropy, > >> as we typically only feed the entropy pool by way of input layer > >> events, a few NIC driver interrupts and disk activity. A non-busy > >> server can easily become entropy-starved. We can mitigate this > >> somewhat by periodically mixing in entropy data based on the > >> delta between multiple high-resolution clocksource reads, per: > >> > >> > >> https://www.osadl.org/Analysis-of-inherent-randomness-of-the-L.rtlws11-developers-okech.0.html > >> > >> Additionally, NIST already approves of similar implementations, so > >> this should be usable in high-securtiy deployments requiring a > >> fair chunk of available entropy data for frequent use of /dev/random. > > > > So, mixed feelings here: > > > > Yes: it's a great idea to regularly mix other data into the pool. More > > samples are always better for RNG quality. > > > > Maybe: the current RNG is not really designed with high-bandwidth > > entropy sources in mind, so this might introduce non-negligible overhead > > in systems with, for instance, huge numbers of CPUs. > > The current implementation is opt-in, and single-threaded, so at least > currently, I don't think there should be any significant issues. But > yeah, there's nothing currently in the implementation preventing a > variant that is per-cpu, which could certainly lead to some scalability > issues.
The pool itself is single-threaded. On large-ish machines (100+ CPUs), we've seen contention rise to 60% or more. Hence the addition of the trickle threshold. But I can see that breaking down with a lot more writers. > > No: it's not a great idea to _credit_ the entropy count with this data. > > Someone watching the TSC or HPET from userspace can guess when samples > > are added by watching for drop-outs in their sampling (ie classic timing > > attack). > > I'm admittedly a bit of a novice in this area... Why does it matter if > someone watching knows more or less when a sample is added? It doesn't > really reveal anything about the sample itself, if we're using a > high-granularity counter value's low bits -- round-trip to userspace has > all sorts of inherent timing jitter, so determining the low-order bits > the kernel got by monitoring from userspace should be more or less > impossible. And the pool is constantly changing, making it a less static > target on an otherwise mostly idle system. I recommend you do some Google searches for "ssl timing attack" and "aes timing attack" to get a feel for the kind of seemingly impossible things that can be done and thereby recalibrate your scale of the impossible. > > (I see you do credit only 1 bit per byte: that's fairly conservative, > > true, but it must be _perfectly conservative_ for the theoretical > > requirements of /dev/random to be met. These requirements are in fact > > known to be unfulfillable in practice(!), but that doesn't mean we > > should introduce more users of entropy accounting. Instead, it means > > that entropy accounting is broken and needs to be removed.) > > Hrm. The government seems to have a different opinion. Various certs > have requirements for some sort of entropy accounting and minimum > estimated entropy guarantees. We can certainly be even more conservative > than 1 bit per byte, but yeah, I don't really have a good answer for > perfectly conservative, and I don't know what might result (on the > government cert front) from removing entropy accounting altogether... Well, the deal with accounting is this: if you earn $.90 and spend $1.00 every day, you'll eventually go broke, even if your rounded-to-the-nearest-dollar accounting tells you you're solidly in the black. The only distinction between /dev/random and urandom is that we claim that /dev/random is always solidly in the black. But as we don't have a firm theoretical basis for making our accounting estimates on the input side, the whole accounting thing kind of breaks down into a kind of busted rate-limiter. We'd do better counting a raw number of samples per source, and then claiming that we've reached a 'full' state when we reach a certain 'diversity x depth' score. And then assuring we have a lot of diversity and depth going into the pool. > Any thoughts on the idea of mixing clocksource bits with reads from > ansi_cprng? Useless. The definition of entropy here can be thought of as 'log(volume of state space that can't be observed by an attacker)'. There is nothing we can do algorithmically to a sample that will increase that volume, we can only shrink it! And any function that is not completely reversible (aka 1:1) will in fact shrink that volume, so you have to be very careful here. The mixing primitives are already quite solid, no need to layer on more potentially faulty ones. -- Mathematics is the supreme nostalgia of our time. -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
