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

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Message-ID: <20220619165615.GB3362@bug>
Date:   Sun, 19 Jun 2022 18:56:15 +0200
From:   Pavel Machek <pavel@....cz>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Eric Biggers <ebiggers@...nel.org>, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Theodore Ts'o <tytso@....edu>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] random: allow writes to /dev/urandom to influence fast
 init

Hi!
> > On Tue, Mar 22, 2022 at 01:14:36PM -0600, Jason A. Donenfeld wrote:
> >> For as far back as I can tell, writing to /dev/urandom or /dev/random
> >> will put entropy into the pool, but won't immediately use it, and won't
> >> credit it either.
> >
> > Did you check kernels v4.7 and earlier?  It looks like this actually changed
> > in
> > v4.8 when the ChaCha20 CRNG was introduced.  v4.7 would mix the data written
> > to
> > /dev/{u,}random into {non,}blocking_pool, which would immediately be
> > reflected
> > in reads from /dev/{u,}random, sys_getrandom(), and get_random_bytes().
> > Writes
> > to /dev/{u,}random didn't affect the input_pool, which was separate.
> 
> Oh, I suppose you might be right, actually, that v4.7 and below would
> hash the non blocking pool, and let /dev/urandom write directly into
> it, as something distinct from the input pool. This changed with v4.8,
> 6 years ago, and now there are no LTS kernels that old, with most
> small devices even having vendor kernels v4.9+. v4.8 apparently did

We are still maintaining 4.4 for -cip project, and people running android probably still 
maintain that, too.

> this while fixing a more extreme vulnerability of allowing unprivileged users to 
> bruteforce input bytes (in addition to allowing unbounded unprivileged lock contention). 

I assume this got fixed during the 4.4-stable series?

Best regards,
										Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html