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

[Jason Cooper <jason@lakedaemon.net>]

On Mon, Feb 17, 2014 at 05:13:07PM +0100, Arnd Bergmann wrote:
> On Monday 17 February 2014 15:54:19 Grant Likely wrote:
> > On Wed, 12 Feb 2014 11:20:00 -0700, Jason Gunthorpe <jgunthorpe@obsidianresearch.com> wrote:
> > > On Wed, Feb 12, 2014 at 12:45:54PM -0500, Jason Cooper wrote:
> > > 
> > > > 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.
> > > 
> > > Sure is a neat idea, but I think in general it would probably be smart
> > > to include the entire FDT blob in the early random pool, that way you
> > > get MACs and other machine unique data too.
> > 
> > I applied a patch that did exactly that (109b623629), and then reverted
> > it (b920ecc82) shortly thereafter because add_device_randomness() is
> > a rather slow function and FDTs can get large. I'd like to see someone
> > do a reasonable analysis on the cost of using an FDT for randomness
> > before I reapply a patch doing something similar. An awful lot of the
> > FDT data is not very random, but there are certainly portions of it that
> > are appropriate for the random pool.
> 
> Could we use a faster hash function that scans the entire device tree and
> then just feed the output of that into add_device_randomness? We probably
> can't expect that there is a lot of entropy in the DT blob, so the
> result wouldn't be all that different in terms of quality of the random
> seed.

I think it would be easier to identify the few attributes that differ
from board to board (mac address, serial number, etc), and differ from
boot to boot (random-seed, timestamp) and just extract and feed those
in.

We already have the fdt parsing code in the decompressor...

thx,

Jason.