Re: [RFC/PATCH 0/3] Add devicetree scanning for randomness

[Olof Johansson]

On Wed, Feb 12, 2014 at 9:45 AM, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:

> I brought this up at last weeks devicetree irc meeting.  My goal is to
> provide early randomness for kaslr on ARM.  Currently, my idea is modify
> the init script to save an additional random seed from /dev/urandom to
> /boot/random-seed.
>
> The bootloader would then load this file into ram, and pass the
> address/size to the kernel either via dt, or commandline.  kaslr (run in
> the decompressor) would consume some of this randomness, and then
> random.c would consume the rest in a non-crediting initialization.
>
> While not ideal, it works in absence of an HRNG, and is no worse than
> the current situation of storing the seed in /var/lib/misc/random-seed.
> It also doesn't require modification of the bootloaders.  Just an
> updated kernel, and update the bootloader environment to load the
> seed.

Hmm. There are some drawbacks with this -- it assumes you can "just
update the bootloader environment" which in general isn't easy to do.
Also, you can't assume that /boot is writable or exists on all
embedded systems.

In general, taking both runtime and system-dependend data and using
that to see entropy is a good idea. For example, device trees that
contain serial numbers and mac addresses for the individual system. I
think x86 feeds the DMI table in for similar purposes.

If that can be amended on some systems with a runtime seed (from
/boot), that's good but we can't rely on it.


-Olof
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