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

[Jason Cooper <jason@lakedaemon.net>]

On Wed, Feb 12, 2014 at 08:12:23PM +0100, Arnd Bergmann wrote:
> On Wednesday 12 February 2014 13:45:21 Jason Cooper wrote:
> > 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.
> 
> Ah, so you are interested in boot loaders that can be scripted to do
> what you had in mind but cannot be scripted to add or modify a DT
> property. I hadn't considered that, but you are probably right that
> this is at least 90% of the systems you'd find in the wild today.

Yes, exactly.

> Thinking this a bit further, I wonder if (at least upstream) u-boot
> has a way to modify DT properties in a scripted way that would allow
> the direct property. It sounds like a generally useful feature not
> just for randomness, so if that doesn't already work, maybe someone
> can implement it. In the simplest case, you'd only need to find the
> address of an existing property in the dtb and load a file to
> that location.

Like fdtget from dtc?  The thing is, this address would only need to be
setup once per board.  Hypothetically, debian's flash-kernel could set
the address in the dtb, then set it in the u-boot environment.  From
then on, only the normal initscript writing to random-seed would be
needed.

thx,

Jason.