Re: [kernel-hardening] Re: [PATCH v2 1/3] Add the latent_entropy gcc plugin

Theodore Ts'o
2016-06-09T19:56:04Z

On Thu, Jun 09, 2016 at 07:22:29PM +0200, PaX Team wrote:
> > Well, the attacker can't control when the interrupts happen, but it
> > could try to burn power by simply having a thread spin in an infinite
> > loop ("0: jmp 0"), sure.
>
> yes, that's one obvious way to accomplish it but even normal applications can
> behave in a similar way, think about spinning event loops, media decoding, etc
> whose sampled insn ptrs may provide less entropy than they get credited for.

Sure, as long as we're assuming less than one bit of entropy per
interrupt, even for a loop which which is:

1: cmpl $1, -8(%rsp)
jz 1b

there would still be *some* uncertainty. And with an event loop there
would be more instructions to sample. Granted, the number of cycles
spent in each will be different, so there will be some biasing, but
that's one of the reason why we've been using 1/64 bit per interrupt.

> yes, no entropy is credited since i don't know how much there is and i tend to err
> on the side of safety which means crediting 0 entropy for latent entropy. of course
> the expectation is that it's not actually 0 but to prove any specific value or limit
> is beyond my skills at least.

Sure, that's fair.

> i think it's not just per 64 interrupts but also after each elapsed second (i.e.,
> whichever condition occurs first), so on an idle system (which i believe is more
> likely to occur on exactly those small systems that the referenced paper was concerned
> about) the credited entropy could be overestimated.

That's a fair concern. It might be that we should enforce some
minimum (at least 8 interrupts in all cases), but this is where it's
all about hueristics, especially on those systems that don't have random_get_entropy().

> > In practice, on most modern CPU where we have a cycle counter,
>
> a quick check for get_cycles shows that at least these archs seem to return 0:
> arc, avr32, cris, frv, m32r, m68k, xtensa. now you may not think of them as modern,
> but they're still used in real life devices. i think that latent entropy is still
> an option on them.

It's possible for a system not to have a cycle counter, but to have
something that can be used instead for random_get_entropy. That's
only being used for the m68k/amiga and mips/R6000[A] cases, but I keep
hoping that the archiecture maintainers for osme of these other
oddball platform (is that better than "non-modern"? :-) will come up
with something, but yes, it is those platforms where I've always been
the most worried. On the one hand, if the hardware is crap, there's
very little you can do. Unfortnuately, very often these crap
architectures have a very low BOM cost, so they are most likely to be
used in IOT devices. :-(

One could try to claim that these IOT devics won't have upgradeable
firmware and, so they'll probably be security disasters even without a
good random number generators, but oddly, that doesn't give me much
solace...

And in the end, that may be the strongest argment for the
latent_entropy plugin. Even if it doesn't provide a lot of extra
entropy, on those platforms we're going to be so starved of real
entropy that almost anything will be better than what we have today.

- Ted