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

[Jason Cooper <jason@lakedaemon.net>]

On Wed, Feb 12, 2014 at 07:17:41PM +0100, Arnd Bergmann wrote:
> On Wednesday 12 February 2014 12:45:54 Jason Cooper 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.
> 
> I like the idea, but wouldn't it be easier to pass actual random data
> using DT, rather than the address/size?

I thought about that at first, but that requires either that the
bootloader be upgraded to insert the data, or that userspace is
modifying the dtb at least twice per boot.

I chose address/size to facilitate modifying existing/fielded devices.
The user could modify the dtb once, and modify the bootloader
environment to load X amount to Y address.  As a fallback, it could be
expressed on the commandline for non-DT bootloaders.

So I'm not against the idea of random-seed,{start,size} and a
random-seed,blob.  I would just like the former to be available for
folks interested in the capability on existing hardware w/o upgrading
the bootloader.

> That way we could even use the same DT binding for passing randomness
> from the bootloader, whereever it may have found that.

The problem lies in defining "whereever" ;)

> If the bootloader has internet connectivity, it could even mix in
> some data from http://www.random.org/cgi-bin/randbyte?nbytes=256&format=f
> ;-)

Gah!  Arnd, you just about gave me a heart attack.  And http no less.

thx,

Jason.