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

[Rob Herring]

On Wed, Feb 12, 2014 at 1:12 PM, Arnd Bergmann <arnd@xxxxxxxx> 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.
>
> 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.

You would be referring to the u-boot fdt command which can read and
set properties. Of course, like all u-boot commands, that may or may
not be enabled by a vendor's u-boot. :(

Rob
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