Re: [PATCH v8 2/3] fdt: add support for rng-seed

[Hsin-Yi Wang]

Then we'd still use add_device_randomness() in case that bootloader
provides weak entropy.

On Tue, Aug 20, 2019 at 7:14 PM Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
>
> On Tue, 20 Aug 2019 at 10:43, Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
> >
> > Hi Ted,
> >
> > Thanks for raising this question.
> >
> > For UEFI based system, they have a config table that carries rng seed
> > and can be passed to device randomness. However, they also use
> > add_device_randomness (not sure if it's the same reason that they
> > can't guarantee _all_ bootloader can be trusted)
>
> The config table is actually a Linux invention: it is populated by the
> EFI stub code (which is part of the kernel) based on the output of a
> call into the EFI_RNG_PROTOCOL, which is defined in the UEFI spec, but
> optional and not widely available.
>
> I have opted for add_device_randomness() since there is no way to
> establish the quality level of the output of EFI_RNG_PROTOCOL, and so
> it is currently only used to prevent the bootup state of the entropy
> pool to be too predictable, and the output does not contribute to the
> entropy estimate kept by the RNG core.
>
>
> > This patch is to let DT based system also have similar features, which
> > can make initial random number stronger. (We only care initial
> > situation here, since more entropy would be added to kernel as time
> > goes on )
> >
> > Conservatively, we can use add_device_randomness() as well, which
> > would pass buffer to crng_slow_load() instead of crng_fast_load().
> > But I think we should trust bootloader here. Whoever wants to use this
> > feature should make sure their bootloader can pass valid (random
> > enough) seeds. If they are not sure, they can just don't add the
> > property to DT.
>
> It is the firmware that adds the property to the DT, not the user.