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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Thu, 24 Mar 2022 08:12:53 -0600
Message-ID: <CAHmME9r4HLCatEi2s5WGsq1GiBPYs=3-SHE4Tw=+9_5u0q25DQ@mail.gmail.com>
Subject: Re: [PATCH] random: allow writes to /dev/urandom to influence fast init
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     David Laight <David.Laight@aculab.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        "Theodore Ts'o" <tytso@mit.edu>, Jann Horn <jannh@google.com>
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Precedence: bulk

Hi Rasmus,

On Wed, Mar 23, 2022 at 2:43 AM Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
>
> 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.

You're not wrong. The actual thing that got committed is:
https://git.buildroot.net/buildroot/commit/?id=f0986de551f46e72268857fd817986e9be697cd0
which thankfully doesn't have this issue.

Jason