[PATCH 1/2] efi: add support for seeding the RNG from a UEFI config table

[Kees Cook]

On Wed, Oct 19, 2016 at 1:18 PM, Ard Biesheuvel
<ard.biesheuvel at linaro.org> wrote:
> On 19 October 2016 at 21:14, Kees Cook <keescook at chromium.org> wrote:
>> On Wed, Oct 19, 2016 at 4:22 AM, Matt Fleming <matt at codeblueprint.co.uk> wrote:
>>> On Wed, 19 Oct, at 12:13:55PM, Ard Biesheuvel wrote:
>>>> On 19 October 2016 at 12:09, Mark Rutland <mark.rutland at arm.com> wrote:
>>>>
>>>> > I think to some extent this mush be treated as an ABI, given cases like
>>>> > kexec.
>>>> >
>>>>
>>>> Perhaps, yes. That would also allow GRUB or other EFI aware
>>>> bootloaders to generate the seed.
>>>
>>> If we're going to go down this route, we should try and get the GUID
>>> into the UEFI spec.
>>
>> It seems like maybe under UEFI, both this table (which sounds like
>> it'll not be rotated regularly)
>
> What do you mean 'rotated'? It is generated at boot. My 2/2 patch
> generates it from the stub using the EFI_RNG_PROTOCOL on ARM/arm64

Oh! I entirely misunderstood. I thought doing regular writes to EFI
variables was discouraged (since they may be stored in NVRAM that
would "wear out").

>> could be mixed with calls to
>> EFI_PROTOCOL_RNG by the kernel? (Similar to how kaslr is seeded?)
>>
>
> That is kind of the point. KASLR is different because we need the
> entropy before even jumping to C code, but for all other uses of early
> entropy, this seemed like a useful approach

Yup, cool. If the table changes per boot, yeah, my suggestion is pointless. :)

-Kees

-- 
Kees Cook
Nexus Security