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

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Date:   Sat, 15 Jul 2017 13:42:15 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     "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

(+ 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.


> diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
> index dee3f5d9df26..0cdb43b268e5 100644
> --- a/Documentation/devicetree/bindings/chosen.txt
> +++ b/Documentation/devicetree/bindings/chosen.txt
> @@ -5,9 +5,27 @@ The chosen node does not represent a real device, but serves as a place
>  for passing data between firmware and the operating system, like boot
>  arguments. Data in the chosen node does not represent the hardware.
>
> +The following properties are recognized:
>
> -stdout-path property
> ---------------------
> +
> +kaslr-seed
> +-----------
> +
> +This property is used when booting with CONFIG_RANDOMIZE_BASE to seed
> +the entropy used to randomize the kernel image base address location. It
> +is parsed as a u64 value, e.g.
> +
> +/ {
> +       chosen {
> +               kaslr-seed = <0xfeedbeef 0xc0def00d>;
> +       };
> +};
> +
> +Note that when booting through EFI when EFI_RNG_PROTOCOL is supported,
> +this value will be overwritten by the EFI stub.
> +
> +stdout-path
> +-----------
>
>  Device trees may specify the device to be used for boot console output
>  with a stdout-path property under /chosen, as described in the Devicetree
> --
> 2.7.4
>
>
> --
> Kees Cook
> Pixel Security