Date:   Fri, 21 Jul 2017 10:57:11 +0200
From:   Stephan Müller <smueller@...onox.de>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Jason A. Donenfeld" <jason@...c4.com>,
        Arnd Bergmann <arnd@...db.de>, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v12 3/4] Linux Random Number Generator

Am Freitag, 21. Juli 2017, 05:08:47 CEST schrieb Theodore Ts'o:

Hi Theodore,

> On Thu, Jul 20, 2017 at 09:00:02PM +0200, Stephan Müller wrote:
> > I concur with your rationale where de-facto the correlation is effect is
> > diminished and eliminated with the fast_pool and the minimal entropy
> > estimation of interrupts.
> > 
> > But it does not address my concern. Maybe I was not clear, please allow me
> > to explain it again.
> > 
> > We have lots of entropy in the system which is discarded by the
> > aforementioned approach (if a high-res timer is present -- without it all
> > bets are off anyway and this should be covered in a separate discussion).
> > At boot time, this issue is fixed by injecting 256 interrupts in the CRNG
> > and consider it seeded.
> > 
> > But at runtime, were we still need entropy to reseed the CRNG and to
> > supply / dev/random. The accounting of entropy at runtime is much too
> > conservative...
> Practically no one uses /dev/random.  It's essentially a deprecated
> interface; the primary interfaces that have been recommended for well
> over a decade is /dev/urandom, and now, getrandom(2).  We only need
> 384 bits of randomness every 5 minutes to reseed the CRNG, and that's
> plenty even given the very conservative entropy estimation currently
> being used.

On a headless system with SSDs, this is not enough based on measurements where 
entropy_avail is always low ...
> 
> This was deliberate.  I care a lot more that we get the initial
> boot-time CRNG initialization right on ARM32 and MIPS embedded
> devices, far, far, more than I care about making plenty of
> information-theoretic entropy available at /dev/random on an x86
> system.  Further, I haven't seen an argument for the use case where
> this would be valuable.

My concern covers *both* /dev/random and /dev/urandom.
> 
> If you don't think they count because ARM32 and MIPS don't have a
> high-res timer, then you have very different priorities than I do.  I
> will point out that numerically there are huge number of these devices
> --- and very, very few users of /dev/random.

With only jiffies, you will not get suitable entropy from interrupts or HID or 
block devices, as these actions can be monitored from user space with a 
suitable degree of precision.

The only real entropy provider would be HID as the entropy may come from the 
key strokes. But that is observable and should not count as entropy.
> 
> > You mentioned that you are super conservative for interrupts due to timer
> > interrupts. In all measurements on the different systems I conducted, I
> > have not seen that the timer triggers an interrupt picked up by
> > add_interrupt_randomness.
> 
> Um, the timer is the largest number of interrupts on my system.  Compare:
> 
>             CPU0       CPU1       CPU2       CPU3
>  LOC:    6396552    6038865    6558646    6057102   Local timer interrupts
> 
> with the number of disk related interrupts:
> 
>  120:      21492     139284      40513    1705886   PCI-MSI 376832-edge     
> ahci[0000:00:17.0]

They seem to be not picked up with the add_interrupt_randomness function.

Execute the follwing SystemTap script:

global NUMSAMPLES = 10000;

global num_events = 0;

probe kernel.function("add_interrupt_randomness")
{
        printf("%d\n", $irq);
        num_events++;

        if (num_events > NUMSAMPLES)
                exit();
}

The timer interrupt does not show up here.

> 
> ... and add_interrupt_randomness() gets called for **every**
> interrupt.  On an mostly idle machine (I was in meetings most of
> today) it's not surprising that time interrupts dominate.  That
> doesn't matter for me as much because I don't really care about
> /dev/random performance.  What's is **far** more important is that the
> entropy estimations behave correctly, across all of Linux's
> architectures, while the kernel is going through startup, before CRNG
> is declared initialized.
> 
> > As we have no formal model about entropy to begin with, we can only assume
> > and hope we underestimate entropy with the entropy heuristic.
> 
> Yes, and that's why I use an ultra-conservative estimate.  If we start
> using a more aggressive hueristic, we open ourselves up to potentially
> very severe security bugs --- and for what?  What's the cost benefit
> ratio here which makes this a worthwhile thing to risk?

The benefit is that in case there is an entropy hog on the system, /dev/random 
and /dev/urandom recover faster from that. Otherwise they do not get reseeded 
at all.
> 
> > Finally, I still think it is helpful to allow (not mandate) to involve the
> > kernel crypto API for the DRNG maintenance (i.e. the supplier for
> > /dev/random and /dev/urandom). The reason is that now more and more DRNG
> > implementations in hardware pop up. Why not allowing them to be used.
> > I.e. random.c would only contain the logic to manage entropy but uses the
> > DRNG requested by a user.
> We *do* allow them to be used.  And we support a large number of
> hardware random number generators already.  See drivers/char/hw_random.

These are noise sources with RNGs, they are not the pure DRNGs without noise 
source I am talking about.
> 
> BTW, I theorize that this is why the companies that could do the
> bootloader random seen work haven't bothered.  Most of their products
> have a TPM or equivalent, and with modern kernel the hw_random
> interface now has a kernel thread that will automatically fill the
> /dev/random entropy pool from the hw_random device.  So this all works
> already, today, without needing a userspace rngd (which used to be
> required).

I am not talking about the input to /dev/random or /dev/urandom. I am talking 
about the DRNG generating the output of /dev/random and /dev/urandom.
> 
> > In addition allowing a replacement of the DRNG component (at compile time
> > at least) may get us away from having a separate DRNG solution in the
> > kernel crypto API. Some users want their chosen or a standardized DRNG to
> > deliver random numbers. Thus, we have several DRNGs in the kernel crypto
> > API which are seeded by get_random_bytes. Or in user space, many folks
> > need their own DRNG in user space in addition to the kernel. IMHO this is
> > all a waste. If we could use the user-requested DRNG when producing
> > random numbers for get_random_bytes or /dev/urandom or getrandom.
> 
> To be honest, I've never understood why that's there in the crypto API
> at all.

Exactly because some folks require a DRNG that meets certain criteria. The 
DRNGs in the kernel crypto API are seeded by get_random_bytes and then produce 
output for the callers just because get_random_bytes has a DRNG for outputting 
data that is not suitable in their eyes.

If get_random_bytes (or the user space interfaces) would be more flexible in 
the output DRNG, the entire business with DRNGs in the kernel crypto API could 
go away.

> But adding more ways to switch out the DRNG for /dev/random
> doesn't solve that problem; in fact it's moving things in the wrong
> direction.

I am always talking about /dev/random and /dev/urandom (as well as 
get_random_bytes).
> 
> Cheers,
> 
> 						- Ted



Ciao
Stephan

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