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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 23 Sep 2019 11:33:21 -0700
Message-ID: <CALCETrWM9opVj+BBrHnnTakTLunW_fB9RM+VSNpNSkR9drDjMw@mail.gmail.com>
Subject: Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and
 introduce getrandom2()
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        "Ahmed S. Darwish" <darwish.07@gmail.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "Alexander E. Patrakov" <patrakov@gmail.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Willy Tarreau <w@1wt.eu>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>
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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.