[Pkg-sysvinit-devel] Bug#587665: Bug#587665: Safety of early boot init of /dev/random seed
Henrique de Moraes Holschuh
hmh at debian.org
Sat Jul 3 16:08:20 UTC 2010
(adding Petter Reinholdtsen to CC, stupid MUA...)
On Sat, 03 Jul 2010, Henrique de Moraes Holschuh wrote:
> 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
> 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.
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