Message-ID: <20100703151636.GB23648@khazad-dum.debian.net>
Date:	Sat, 3 Jul 2010 12:16:36 -0300
From:	Henrique de Moraes Holschuh <hmh@...ian.org>
To:	Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Theodore Ts'o <tytso@....edu>
Cc:	Petter Reinholdtsen <pere@...gry.com>, 587665@...s.debian.org,
	Michael Biebl <biebl@...ian.org>, linux-kernel@...r.kernel.org
Subject: Safety of early boot init of /dev/random seed

Hello,

We are trying to enhance the Debian support for /dev/random seeding at early
boot, and we need some expert help to do it right.  Maybe some of you could
give us some enlightenment on a few issues?

Apologies in advance if I got the list of Linux kernel maintainers wrong.  I
have also copied LKML just in case.

A bit of context:  Debian tries to initialize /dev/random, by restoring the
pool size and giving it some seed material (through a write to /dev/random)
from saved state stored in /var.

Since we store the seed data in /var, that means we only feed it to
/dev/random relatively late in the boot sequence, after remote filesystems
are available.  Thus, anything that needs random numbers earlier than that
point will run with whatever the kernel managed to harness without any sort
of userspace help (which is probably not much, especially on platforms that
clear RAM contents at reboot, or after a cold boot).

We take care of regenerating the stored seed data as soon as we use it, in
order to avoid as much as possible the possibility of reuse of seed data.
This means that we write the old seed data to /dev/random, and immediately
copy poolsize bytes from /dev/urandom to the seed data file.

The seed data file is also regenerated prior to shutdown.

We would like to clarify some points, so as to know how safe they are on
face of certain error modes, and also whether some of what we do is
necessary at all.  Unfortunately, real answers require more intimate
knowledge of the theory behind Linux' random pools than we have in the
Debian initscripts team.

Here are our questions:

1. How much data of unknown quality can we feed the random pool at boot,
   before it causes damage (i.e. what is the threshold where we violate the
   "you are not goint to be any worse than you were before" rule) ?

2. How dangerous it is to feed the pool with stale seed data in the next
   boot (i.e. in a failure mode where we do not regenerate the seed file) ?

3. What is the optimal size of the seed data based on the pool size ?

4. How dangerous it is to have functions that need randomness (like
   encripted network and partitions, possibly encripted swap with an
   ephemeral key), BEFORE initializing the random seed ?

5. Is there an optimal size for the pool?  Does the quality of the randomness
   one extracts from the pool increase or decrease with pool size?

Basically, we need these answers to find our way regarding the following
decisions:

a) Is it better to seed the pool as early as possible and risk a larger time
   window for problem (2) above, instead of the current behaviour where we
   have a large time window where (4) above happens?

b) Is it worth the effort to base the seed file on the size of the pool,
   instead of just using a constant size?  If a constant size is better,
   which size would that be? 512 bytes? 4096 bytes? 16384 bytes?

c) What is the maximum seed file size we can allow (maybe based on size of
   the pool) to try to avoid problem (1) above ?

We would be very grateful if you could help us find good answers to the
questions above.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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