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

Ahmed S. Darwish
2019-09-26T21:12:07Z

On Mon, Sep 23, 2019 at 11:33:21AM -0700, Andy Lutomirski wrote:
> On Fri, Sep 20, 2019 at 11:07 PM Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * Linus Torvalds:
> >
> > > Violently agreed. And that's kind of what the GRND_EXPLICIT is really
> > > aiming for.
> > >
> > > However, it's worth noting that nobody should ever use GRND_EXPLICIT
> > > directly. That's just the name for the bit. The actual users would use
> > > GRND_INSECURE or GRND_SECURE.
> >
> > Should we switch glibc's getentropy to GRND_EXPLICIT? Or something
> > else?
> >
> > I don't think we want to print a kernel warning for this function.
> >
>
> Contemplating this question, I think the answer is that we should just
> not introduce GRND_EXPLICIT or anything like it. glibc is going to
> have to do *something*, and getentropy() is unlikely to just go away.
> The explicitly documented semantics are that it blocks if the RNG
> isn't seeded.
>
> Similarly, FreeBSD has getrandom():
>
> https://www.freebsd.org/cgi/man.cgi?query=getrandom&sektion=2&manpath=freebsd-release-ports
>
> and if we make getrandom(..., 0) warn, then we have a situation where
> the *correct* (if regrettable) way to use the function on FreeBSD
> causes a warning on Linux.
>
> Let's just add GRND_INSECURE, make the blocking mode work better, and,
> if we're feeling a bit more adventurous, add GRND_SECURE_BLOCKING as a
> better replacement for 0, ...

This is what's now done in the just-submitted V5, except the "make the
blocking mode work better" part:

https://lkml.kernel.org/r/20190926204217.GA1366@pc

It's a very conservative patch so far IMHO (minus the loud warning).

Thanks,
--
Ahmed Darwish