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

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Date:   Mon, 17 Jul 2017 12:56:10 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Kees Cook <keescook@...omium.org>,
        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 Sun, Jul 16, 2017 at 05:42:25PM +0100, Ard Biesheuvel wrote:
> 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.

I was under the impression that we'd already assumed other bootloaders could
set this, so I don't have a problem promoting this to a defined public
interface.

I guess we just need Will and Catalin to agree.

FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

As an aside, we might want to make a split between /chosen properties which are
Linux-specific (e.g. this), and those which are somewhat generic (e.g.
stdout-path), since other OSs may/should respect those generic ones.

Mark.