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

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Message-ID: <3707a8f9-93e7-ee54-42a3-ac12a279c6bc@rasmusvillemoes.dk>
Date:   Wed, 23 Mar 2022 09:43:28 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        David Laight <David.Laight@...LAB.COM>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-crypto@...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

On 23/03/2022 03.50, Jason A. Donenfeld wrote:

> - Since these seeding shell scripts have always been broken, because
>   this is how the rng has always been, rather than trying to bolt on a
>   very imperfect fix in the kernel for something that never worked
>   right, we could suggest shell scripts take the path that I implemented
>   for systemd:
>   https://github.com/systemd/systemd/commit/da2862ef06f22fc8d31dafced6d2d6dc14f2ee0b
>   In shell, this would look like:
> 
>     #!/bin/bash
>     cat seedfile > /dev/urandom
>     { cat seedfile; head -c 32 /dev/urandom; } | sha256sum | cut -d ' ' -f 1 > seedfile

Maybe stating the obvious, but in the interest of preventing
proliferation of more broken shell scripts: The tail of the above should
be spelled

  ...  > seedfile.tmp && mv seedfile.tmp seedfile

or seedfile would be truncated before cat had a chance to read it.

Rasmus