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

[Austin S. Hemmelgarn]

On 2016-06-21 09:20, Stephan Mueller wrote:
> 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.
That would be the ones which list 'QEMU Virtual CPU version X.Y' as the 
CPU string.  The only things that return that in the CPUID data are 
either QEMU itself, or software that is based on QEMU.
> > 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.
Talking about the common case is all well and good, but the lower bound 
still needs to be taken into account.  If the test results aren't 
uniformly distributed within that interval, or even following a typical 
Gaussian distribution within it (which is what I and many other people 
would probably assume without the data later in the appendix), then you 
really need to mention this _before_ the table itself.  Such information 
is very important, and not everyone has time to read everything.
> 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.
Apologies for not reading that part thoroughly, you might want to add 
those results to the table too.
> > 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.
Then you shouldn't be pushing the documentation as what appears to be 
your sole argument for including it 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.
I'm not complaining as much about the lack of data for such devices as I 
am about you stating that it will work fine for such devices when you 
have so little data to support those claims.  Many of the devices you 
have listed that can be reasonably assumed to be embedded systems are 
relatively modern ones that most people would think of (smart-phones and 
similar).  Such systems have almost as much if not more interrupts as 
many desktop and server systems, so the entropy values there actually do 
make some sense.  Not everything has this luxury.  Think for example of 
a router.  All it will generally have interrupts from is the timer 
interrupt (which should functionally have near zero entropy because it's 
monotonic most of the time) and the networking hardware, and quite 
often, many of the good routers operate their NIC's in polling mode, 
which means very few interrupts (which indirectly is part of the issue 
with some server systems too), and therefore will have little to no 
entropy there either.  This is an issue with the current system too, but 
you have almost zero data on such systems systems yourself, so you can't 
argue that it makes things better for them.
> 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.
I would love to, but sadly the only system I have that isn't an x86 box 
that actually boots at all right now is an almost 2 decade old PA-RISC 
box that barely runs Linux, and the number of people who would be 
interested in the results from that can probably be counted on one hand.