Re: [PATCH v4 0/5] /dev/random - a new approach

[Stephan Mueller]

Am Dienstag, 21. Juni 2016, 09:05:55 schrieb Austin S. Hemmelgarn:

Hi Austin,

> On 2016-06-20 14:32, Stephan Mueller wrote:
> > Am Montag, 20. Juni 2016, 13:07:32 schrieb Austin S. Hemmelgarn:
> > 
> > Hi Austin,
> > 
> >> On 2016-06-18 12:31, Stephan Mueller wrote:
> >>> Am Samstag, 18. Juni 2016, 10:44:08 schrieb Theodore Ts'o:
> >>> 
> >>> Hi Theodore,
> >>> 
> >>>> At the end of the day, with these devices you really badly need a
> >>>> hardware RNG.  We can't generate randomness out of thin air.  The only
> >>>> thing you really can do requires user space help, which is to generate
> >>>> keys lazily, or as late as possible, so you can gather as much entropy
> >>>> as you can --- and to feed in measurements from the WiFi (RSSI
> >>>> measurements, MAC addresses seen, etc.)  This won't help much if you
> >>>> have an FBI van parked outside your house trying to carry out a
> >>>> TEMPEST attack, but hopefully it provides some protection against a
> >>>> remote attacker who isn't try to carry out an on-premises attack.
> >>> 
> >>> All my measurements on such small systems like MIPS or smaller/older
> >>> ARMs
> >>> do not seem to support that statement :-)
> >> 
> >> Was this on real hardware, or in a virtual machine/emulator?  Because if
> >> it's not on real hardware, you're harvesting entropy from the host
> >> system, not the emulated one.  While I haven't done this with MIPS or
> >> ARM systems, I've taken similar measurements on SPARC64, x86_64, and
> >> PPC64 systems comparing real hardware and emulated hardware, and the
> >> emulated hardware _always_ has higher entropy, even when running the
> >> emulator on an identical CPU to the one being emulated and using KVM
> >> acceleration and passing through all the devices possible.
> >> 
> >> Even if you were testing on real hardware, I'm still rather dubious, as
> >> every single test I've ever done on any hardware (SPARC, PPC, x86, ARM,
> >> and even PA-RISC) indicates that you can't harvest entropy as
> >> effectively from a smaller CPU compared to a large one, and this effect
> >> is significantly more pronounced on RISC systems.
> > 
> > It was on real hardware. As part of my Jitter RNG project, I tested all
> > major CPUs from small to big -- see Appendix F [1]. For MIPS/ARM, see the
> > trailing part of the big table.
> > 
> > [1] http://www.chronox.de/jent/doc/CPU-Jitter-NPTRNG.pdf
> 
> Specific things I notice about this:
> 1. QEMU systems are reporting higher values than almost anything else
> with the same ISA.  This makes sense, but you don't appear to have
> accounted for the fact that you can't trust almost any of the entropy in
> a VM unless you have absolute trust in the host system, because the host
> system can do whatever the hell it wants to you, including manipulating
> timings directly (with a little patience and some time spent working on
> it, you could probably get those number to show whatever you want just
> by manipulating scheduling parameters on the host OS for the VM software).

I am not sure where you see QEMU systems listed there.

> 2. Quite a few systems have a rather distressingly low lower bound and
> still get accepted by your algorithm (a number of the S/390 systems, and
> a handful of the AMD processors in particular).

I am aware of that, but please read the entire documentation where the lower 
and upper boundary comes from and how the Jitter RNG really operates. There 
you will see that the lower boundary is just that: it will not be lower, but 
the common case is the upper boundary.

Furthermore, the use case of the Jitter RNG is to support the DRBG seeding 
with a very high reseed interval.

> 3. Your statement at the bottom of the table that 'all test systems at
> least un-optimized have a lower bound of 1 bit' is refuted by your own
> data, I count at least 2 data points where this is not the case.  One of
> them is mentioned at the bottom as an outlier, and you have data to back
> this up listed in the table, but the other (MIPS 4Kec v4.8) is the only
> system of that specific type that you tested, and thus can't be claimed
> as an outlier.

You are right, I have added more and more test results to the table without 
updating the statement below. I will fix that.

But note, that there is a list below that statement providing explanations 
already. So, it is just that one statement that needs updating.

> 4. You state the S/390 systems gave different results when run
> un-optimized, but don't provide any data regarding this.

The pointer to appendix F.46 was supposed to cover that issue.

> 5. You discount the Pentium Celeron Mobile CPU as old and therefore not
> worth worrying about.  Linux still runs on 80486 and other 'ancient'
> systems, and there are people using it on such systems.  You need to
> account for this usage.

I do not account for that in the documentation. In real life though, I 
certainly do -- see how the Jitter RNG is used in the kernel.

> 6. You have a significant lack of data regarding embedded systems, which
> is one of the two biggest segments of Linux's market share.  You list no
> results for any pre-ARMv6 systems (Linux still runs on and is regularly
> used on ARMv4 CPU's, and it's worth also pointing out that the values on
> the ARMv6 systems are themselves below average), any MIPS systems other
> than 24k and 4k (which is not a good representation of modern embedded
> usage), any SPARC CPU's other than UltraSPARC (ideally you should have
> results on at least a couple of LEON systems as well), no tight-embedded
> PPC chips (PPC 440 processors are very widely used, as are the 7xx and
> 970 families, and Freescale's e series), and only one set of results for
> a tight-embedded x86 CPU (the Via Nano, you should ideally also have
> results on things like an Intel Quark).  Overall, your test system
> selection is not entirely representative of actual Linux usage (yeah,
> ther'es a lot of x86 servers out there running Linux, there's at least
> as many embedded systems running it too though, even without including
> Android).

Perfectly valid argument. But I programmed that RNG as a hobby -- I do not 
have the funds to buy all devices there are.

And http://www.chronox.de/jent.html asks for help -- if you have those 
devices, please help and simply execute one application and return the data to 
me.

> 7. The RISC CPU's that you actually tested have more consistency within
> a particular type than the CISC CPU's.  Many of them do have higher
> values than the CISC CPU's, but a majority of the ones I see listed
> which have such high values are either old systems not designed for low
> latency, or relatively big SMP systems (which will have higher entropy
> because of larger numbers of IRQ's, as well as other factors).

Ok, run the tests on the systems you like and return the results to me.


Ciao
Stephan