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

[Ard Biesheuvel]

On Wed, 21 Aug 2019 at 08:57, Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
>
> Then we'd still use add_device_randomness() in case that bootloader
> provides weak entropy.
>

(please don't top post)

Whether to trust the firmware provided entropy is a policy decision,
and typically, we try to avoid dictating policy in the kernel, and
instead, we try to provide a sane default but give the user control
over it.

So in this case, we should probably introduce
add_firmware_randomness() with a Kconfig/cmdline option pair to decide
whether it should be trusted or not (or reuse the one we have for
trusting RDRAND etc)


> 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.