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

Andy Lutomirski
2019-09-23T18:33:40Z

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, convince FreeBSD to add it too, and then
worry about deprecating 0 once we at least get some agreement from the
FreeBSD camp.