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

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Date:   Thu, 26 Sep 2019 23:11:50 +0200
From:   "Ahmed S. Darwish" <darwish.07@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lennart Poettering <mzxreary@...inter.de>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Willy Tarreau <w@....eu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and
 introduce getrandom2()

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@...hat.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