Re: [PATCH] random: allow writes to /dev/urandom to influence fast init

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: David Laight <David.Laight@aculab.com>
Cc: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>,
	Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, Jann Horn <jannh@google.com>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	Guenter Roeck <linux@roeck-us.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"Theodore Ts'o" <tytso@mit.edu>,
	Sandy Harris <sandyinchina@gmail.com>
Subject: Re: [PATCH] random: allow writes to /dev/urandom to influence fast init
Date: Wed, 23 Mar 2022 13:53:03 -0600	[thread overview]
Message-ID: <CAHmME9rxV-WLBCGyMRwba_8nF_onRfBi0+-xz84-HLNfee=R3Q@mail.gmail.com> (raw)
In-Reply-To: <7cde489e73c8448b95a1b7bc6ed1d75b@AcuMS.aculab.com>

Hi David,

On Wed, Mar 23, 2022 at 8:01 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Jason A. Donenfeld
> > Sent: 23 March 2022 04:48
> ...
> > - Plenty of things are seeding the RNG correctly, and buildroot's
> > shell script is just "doing it wrong".
> >
> > On that last point, I should reiterate that buildroot's shell script
> > still isn't actually initializing the RNG, despite what it says in its
> > echo; there's never been a way to initialize the RNG from a shell
> > script, without calling out to various special purpose ioctl-aware
> > binaries.
>
> Perhaps the very first write after boot could be assumed to
> be valid initialisation data?
> (On top of a few other tests.)

I addressed this already earlier. That approach does not work. Too
many things already pass in garbage, not expecting for it to be
credited, but just contributory. /dev/urandom writes simply has never
had the semantics one would want for credited seeding. Adding a
heuristic like this will break users.

Jason