Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and introduce getrandom2()

Ahmed S. Darwish
2019-09-20T13:46:26Z

Hi,

On Wed, Sep 18, 2019 at 04:57:58PM -0700, Linus Torvalds wrote:
> On Wed, Sep 18, 2019 at 2:17 PM Ahmed S. Darwish <darwish.07@gmail.com> wrote:
> >
> > Since Linux v3.17, getrandom(2) has been created as a new and more
> > secure interface for pseudorandom data requests. It attempted to
> > solve three problems, as compared to /dev/urandom:
>
> I don't think your patch is really _wrong_, but I think it's silly to
> introduce a new system call, when we have 30 bits left in the flags of
> the old one, and the old system call checked them.
>
> So it's much simpler and more straightforward to just introduce a
> single new bit #2 that says "I actually know what I'm doing, and I'm
> explicitly asking for secure/insecure random data".
>
> And then say that the existing bit #1 just means "I want to wait for entropy".
>
> So then you end up with this:
>
> /*
> * Flags for getrandom(2)
> *
> * GRND_NONBLOCK Don't block and return EAGAIN instead
> * GRND_WAIT_ENTROPY Explicitly wait for entropy
> * GRND_EXPLICIT Make it clear you know what you are doing
> */
> #define GRND_NONBLOCK 0x0001
> #define GRND_WAIT_ENTROPY 0x0002
> #define GRND_EXPLICIT 0x0004
>
> #define GRND_SECURE (GRND_EXPLICIT | GRND_WAIT_ENTROPY)
> #define GRND_INSECURE (GRND_EXPLICIT | GRND_NONBLOCK)
>
> /* Nobody wants /dev/random behavior, nobody should use it */
> #define GRND_RANDOM 0x0002
>
> which is actually fairly easy to understand. So now we have three
> bits, and the values are:
>
> 000 - ambiguous "secure or just lazy/ignorant"
> 001 - -EAGAIN or secure
> 010 - blocking /dev/random DO NOT USE
> 011 - nonblocking /dev/random DO NOT USE
> 100 - nonsense, returns -EINVAL
> 101 - /dev/urandom without warnings
> 110 - blocking secure
> 111 - -EAGAIN or secure
>

Hmmm, the point of the new syscall was **exactly** to avoid the 2^3
combinations above, and to provide developers only two, sane and easy,
options:

- GRND2_INSECURE
- GRND2_SECURE_UNBOUNDED_INITIAL_WAIT

You *must* pick one of these, and that's it. (!)

Then the proposed getrandom_wait(7) manpage, also mentioned in the V4
patch WARN message, would provide a big rationale, and encourage
everyone to use the new getrandom2(2) syscall instead.

But yeah, maybe we should add the extra flags to the old getrandom()
instead, and let glibc implement a getrandom_safe(3) wrapper only
with the sane options available.

Problem is, glibc is still *really* slow in adopting linux syscall
wrappers, so I'm not optimistic about that...

I still see the new system call as the sanest path, even provided
the cost of a new syscall number..

@Linus, @Ted: Final thoughts?

> and people would be encouraged to use one of these three:
>
> - GRND_INSECURE
> - GRND_SECURE
> - GRND_SECURE | GRND_NONBLOCK
>
> all of which actually make sense, and none of which have any
> ambiguity. And while "GRND_INSECURE | GRND_NONBLOCK" works, it's
> exactly the same as just plain GRND_INSECURE - the point is that it
> doesn't block for entropy anyway, so non-blocking makes no different.
>

[...]

>
> There is *one* other small semantic change: The old code did
> urandom_read() which added warnings, but each warning also _reset_ the
> crng_init_cnt. Until it decided not to warn any more, at which point
> it also stops that resetting of crng_init_cnt.
>
> And that reset of crng_init_cnt, btw, is some cray cray.
>
> It's basically a "we used up entropy" thing, which is very
> questionable to begin with as the whole discussion has shown, but
> since it stops doing it after 10 cases, it's not even good security
> assuming the "use up entropy" case makes sense in the first place.
>
> So I didn't copy that insanity either. And I'm wondering if removing
> it from /dev/urandom might also end up helping Ahmed's case of getting
> entropy earlier, when we don't reset the counter.
>

Yeah, noticed that, but I've learned not to change crypto or
speculative-execution code even if the changes "just look the same" at
first glance ;-)

(out of curiosity, I'll do a quick test with this CRNG entropy reset
part removed. Maybe it was indeed part of the problem..)

> But other than those two details, none of the existing semantics
> changed, we just added the three actually _sane_ cases without any
> ambiguity.
>
> In particular, this still leaves the semantics of that nasty
> "getrandom(0)" as the same "blocking urandom" that it currently is.
> But now it's a separate case, and we can make that perhaps do the
> timeout, or at least the warning.
>

Yeah, I would propose to keep the V4-submitted "timeout then WARN"
logic. This alone will give user-space / distributions time to adapt.

For example, it was interesting that even the 0day bot had limited
entropy on boot (virtio-rng / TRUST_CPU not enabled):

https://lkml.kernel.org/r/20190920005120.GP15734@shao2-debian

If user-space didn't get its act together, then the other extreme
measures can be implemented later (the getrandom() length test, using
jitter as a credited kernel entropy source, etc., etc.)

> And the new cases are defined to *not* warn. In particular,
> GRND_INSECURE very much does *not* warn about early urandom access
> when crng isn't ready. Because the whole point of that new mode is
> that the user knows it isn't secure.
>
> So that should make getrandom(GRND_INSECURE) palatable to the systemd
> kind of use that wanted to avoid the pointless kernel warning.
>

Yup, that's what was in the submitted V4 patch too. The caller
explicitly asked for "insecure", so they know what they're doing.

getrandom2(2) never prints any kernel message.

> And we could mark this for stable and try to get it backported so that
> it will have better coverage, and encourage people to use the new sane
> _explicit_ waiting (or not) for entropy.
>

ACK. I'll wait for an answer to the "Final thoughts?" question above,
send a V5 with CC:stable, then disappear from this thread ;-)

Thanks a lot everyone!

--
Ahmed Darwish