Re: [PATCH] Documentation: dt: chosen property for kaslr-seed

[Ard Biesheuvel]

On 16 July 2017 at 03:13, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Sat, Jul 15, 2017 at 5:42 AM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>> (+ Mark, Will, Catalin)
>>
>> On 15 July 2017 at 01:38, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>> Document then /chosen/kaslr-seed property (and its interaction with the
>>> EFI_RNG_PROTOCOL API).
>>>
>>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>> ---
>>>  Documentation/devicetree/bindings/chosen.txt | 22 ++++++++++++++++++++--
>>>  1 file changed, 20 insertions(+), 2 deletions(-)
>>>
>>
>> For the textual changes:
>>
>> Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>>
>> *However*, documenting the /chosen/kaslr-seed property promotes it
>> from a stub<->kernel private interface to an external ABI between the
>> kernel and the bootloader, and we need to reach agreement on whether
>> doing so is desirable first IMHO.
>>
>
> Oh! I thought that was the point (having a bootloader provide kaslr
> entropy). And that in the EFI case, it was the stub doing it.
>

It was the opposite, actually,  The /chosen node is the most
appropriate way for the EFI stub to communicate a seed value to the
kernel proper, given how it is needed extremely early in the boot.
(Using UEFI config tables like we do for the /dev/random seed is not
possible for this)

So as a side effect, other bootloaders can use the same mechanism. I'm
fine with that, but it needs to be an explicit decision by the
maintainers imo.