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

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Message-ID: <CAKv+Gu82xiv0Yy79Hb=JCqmJNYoxxSt5HRu5j4TeSeznCvg_+w@mail.gmail.com>
Date:   Sun, 16 Jul 2017 17:42:25 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Matt Redfearn <matt.redfearn@...tec.com>
Subject: Re: [PATCH] Documentation: dt: chosen property for kaslr-seed

On 16 July 2017 at 03:13, Kees Cook <keescook@...omium.org> wrote:
> On Sat, Jul 15, 2017 at 5:42 AM, Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
>> (+ Mark, Will, Catalin)
>>
>> On 15 July 2017 at 01:38, Kees Cook <keescook@...omium.org> wrote:
>>> Document then /chosen/kaslr-seed property (and its interaction with the
>>> EFI_RNG_PROTOCOL API).
>>>
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>> ---
>>>  Documentation/devicetree/bindings/chosen.txt | 22 ++++++++++++++++++++--
>>>  1 file changed, 20 insertions(+), 2 deletions(-)
>>>
>>
>> For the textual changes:
>>
>> Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>>
>> *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.