Re: [PATCH] hw/arm/virt: dt: add rng-seed property

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Wed, Jun 29, 2022 at 11:18:23AM +0100, Alex Bennée wrote:
> 
> Peter Maydell <peter.maydell@linaro.org> writes:
> 
> > On Tue, 28 Jun 2022 at 19:45, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >>
> >> On 6/27/22, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >> > On 6/27/22, Peter Maydell <peter.maydell@linaro.org> wrote:
> >> >> On Mon, 27 Jun 2022 at 17:07, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >> >>>
> >> >>> In 60592cfed2 ("hw/arm/virt: dt: add kaslr-seed property"), the
> >> >>> kaslr-seed property was added, but the equally as important rng-seed
> >> >>> property was forgotten about, which has identical semantics for a
> >> >>> similar purpose. This commit implements it in exactly the same way as
> >> >>> kaslr-seed.
> >> >>
> >> >> Not an objection, since if this is what the dtb spec says we need
> >> >> to provide then I guess we need to provide it, but:
> >> >> Why do we need to give the kernel two separate random seeds?
> >> >> Isn't one sufficient for the kernel to seed its RNG and generate
> >> >> whatever randomness it needs for whatever purposes it wants it?
> >> >>
> >> >
> >> > Seems a bit silly to me too. `rng-seed` alone ought to be sufficient.
> >> > After the kernel calls add_bootloader_randomness() on it,
> >> > get_random_long() can be used for kaslr'ing and everything else too.
> >> > So I'm not sure what's up, but here we are. Maybe down the line I'll
> >> > look into the details and formulate a plan to remove `kaslr-seed` if
> >> > my supposition is correct.
> 
> Sorry now I've had my coffee and read properly I see you are already
> aware of kaslr-seed. However my point about suppression would still
> stand because for the secure boot flow you need checksum-able DTBs.

Please read the patch. Maybe take a sip of coffee first. There's a knob
for this too.

The code is exactly the same for kaslr-seed and rng-seed. Everytime
there's some kaslr-seed thing, there is now the same rng-seed thing.

Jason