[PATCH 1/1] efi_leader: delete rng-seed if having EFI RNG protocol

[Ilias Apalodimas]

On Thu, 19 Sept 2024 at 18:19, Simon Glass <sjg at chromium.org> wrote:
>
> Hi,
>
> On Thu, 19 Sept 2024 at 17:13, Ilias Apalodimas
> <ilias.apalodimas at linaro.org> wrote:
> >
> >
> >
> > On Thu, Sep 19, 2024, 18:05 Heinrich Schuchardt <heinrich.schuchardt at canonical.com> wrote:
> >>
> >> On 19.09.24 17:00, Simon Glass wrote:
> >> > Hi,
> >> >
> >> > On Thu, 19 Sept 2024 at 16:32, Ilias Apalodimas
> >> > <ilias.apalodimas at linaro.org> wrote:
> >> >>
> >> >> Hi all,
> >> >>
> >> >> On Thu, 19 Sept 2024 at 17:20, Heinrich Schuchardt
> >> >> <heinrich.schuchardt at canonical.com> wrote:
> >> >>>
> >> >>> On 19.09.24 16:10, Simon Glass wrote:
> >> >>>> Hi Heinrich,
> >> >>>>
> >> >>>> On Sat, 14 Sept 2024 at 18:06, Heinrich Schuchardt
> >> >>>> <heinrich.schuchardt at canonical.com> wrote:
> >> >>>>>
> >> >>>>> For measured be boot we must avoid any volatile values in the device-tree.
> >> >>>>> We already delete /chosen/kaslr-seed if we provide and EFI RNG protocol.
> >> >>>>
> >> >>>> Could you explain a bit why this is, and where this is checked?
> >> >>>>>
> >> >>>>> Additionally remove /chosen/rng-seed provided by QEMU or U-Boot.
> >> >>>
> >> >>> Measured boot relies on creating hashes of artifacts and writing these
> >> >>> to TPM. If the hashes don't match the OS will either warn or refuse to
> >> >>> boot. The device-tree is one of the artifacts that are measured.
> >> >>>
> >> >>> If we have random values in /chosen, measured boot will fail.
> >> >>>
> >> >>> When an EFI RNG protocol is provided by the firmware, GRUB and the
> >> >>> kernel will use it instead of /chosen/rng-seed and /chosen/kaslr-seed.
> >> >>
> >> >> There's a comment on top of that function that explains what happens as well.
> >> >> In short the EFI stub does not even look at the KASLR seed and never
> >> >> randomizes the physical placement of the kernel. It only does that
> >> >> when the EFI_RNG protocol is there.
> >> >
> >> > OK thank you. I suppose I am more just wondering why it got added in
> >> > the first place?
> >>
> >> For booting via the legacy Linux entry point adding kaslr-seed allows to
> >> randomize addresses. QEMU adds rng-seed instead of kaslr-seed.
> >
> >
> > Not the kernel physical placement. It randomizes only the virtual placement
>
> So, are you saying that U-Boot adds this field into the FDT and then removes it?

Yes. As Heinrich said, the rng seed is still usable for some
randomization. If we boot with EFI and have an RNG protocol, we dont
need it and it also messes up the TPM measurements, so we remove it.
But the code that injects it to u-boot, or the prior bootloader that
handed you over a DT,  does not know if you plan to boot with EFI.

Regards
/Ilias

>
> Regards,
> Simon