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Date:   Mon, 25 Jul 2022 18:51:36 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Rich Felker <dalias@libc.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
        libc-alpha@sourceware.org, Yann Droneaud <ydroneaud@opteya.com>,
        jann@thejh.net, Michael@phoronix.com,
        Paul Eggert <eggert@cs.ucla.edu>, linux-crypto@vger.kernel.org
Subject: Re: arc4random - are you sure we want these?
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Hi Florian,

On Mon, Jul 25, 2022 at 06:40:54PM +0200, Florian Weimer wrote:
> The core issue is that on some kernels/architectures, reading from
> /dev/urandom can degrade to GRND_INSECURE (approximately), and while the
> result is likely still unpredictable, not everyone would label that as a
> CSPRNG.

On some old kernels (though I think not all?), you can poll on
/dev/random. This isn't perfect, as the ancient "non blocking pool"
initialized after the "blocking pool", but it's not too imperfect
either. Take a look at the previously linked random-util.c.

> If we document arc4random as a CSPRNG, this means that we would have to
> ditch the fallback code and abort the process if the getrandom system
> call is not available: when reading from /dev/urandom as a fallback, we
> have no way of knowing if we are in any of the impacted execution
> environments.  Based on your other comments, it seems that you are
> interested in such fallbacks, too, but I don't think you can actually
> have both (CSPRNG + fallback).
> 
> And then there is the certification issue.  We really want applications
> that already use OpenSSL for other cryptography to use RAND_bytes
> instead of arc4random.  Likewise for GNUTLS and gnutls_rnd.  What should
> authors of those cryptographic libraries?  That's less clear, and really
> depends on the constraints they operate in (e.g., they may target only a
> subset of architectures and kernel versions).

I think all of this is yet another indication that there are some major
things to work out -- should we block or not? is buffering safe? is the
interface correct? -- and so we should just back out the arc4random
commit until this has been explored a bit more. We're not gaining
anything from rushing this, especially as a "source code compatibility"
thing, if there's not even agreement between OSes on what the function
does inside.

Jason

PS: please try to keep linux-crypto@vger.kernel.org CC'd. I've been
bouncing these manually when not, but it's hard to keep up with that.