Re: [PATCH RFC v2] random: optionally block in getrandom(2) when the CRNG is uninitialized

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sun, Sep 15, 2019 at 11:32 AM Willy Tarreau <w@1wt.eu> wrote:
>
> I think that the exponential decay will either not be used or
> be totally used, so in practice you'll always end up with 0 or
> 30s depending on the entropy situation

According to the systemd random-seed source snippet that Ahmed posted,
it actually just tries once (well, first once non-blocking, then once
blocking) and then falls back to reading urandom if it fails.

So assuming there's just one of those "read much too early" cases, I
think it actually matters.

But while I tried to test this, on my F30 install, systemd seems to
always just use urandom().

I can trigger the urandom read warning easily enough (turn of CPU
rdrand trusting and increase the entropy requirement by a factor of
ten, and turn of the ioctl to add entropy from user space), just not
the getrandom() blocking case at all.

So presumably that's because I have a systemd that doesn't use
getrandom() at all, or perhaps uses the 'rdrand' instruction directly.
Or maybe because Arch has some other oddity that just triggers the
problem.

> In addition, since you're leaving the door open to bikeshed around
> the timeout valeue, I'd say that while 30s is usually not huge in a
> desktop system's life, it actually is a lot in network environments
> when it delays a switchover.

Oh, absolutely.

But in that situation you have a MIS person on call, and somebody who
can fix it.

It's not like switchovers happen in a vacuum. What we should care
about is that updating a kernel _works_. No regressions. But if you
have some five-nines setup with switchover, you'd better have some
competent MIS people there too. You don't just switch kernels without
testing ;)

                 Linus