Re: [PATCH v2] hw/arm/virt: dt: add rng-seed property

[Peter Maydell]

On Thu, 30 Jun 2022 at 11:39, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> In 60592cfed2 ("hw/arm/virt: dt: add kaslr-seed property"), the
> kaslr-seed property was added, but the equally as important rng-seed
> property was forgotten about, which has identical semantics for a
> similar purpose. This commit implements it in exactly the same way as
> kaslr-seed. It then changes the name of the disabling option to reflect
> that this has more to do with randomness vs determinism, rather than
> something particular about kaslr.
>
> Cc: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Thanks for this respin; I think this is the right way to go.

> ---
>  docs/system/arm/virt.rst | 17 ++++++++++------
>  hw/arm/virt.c            | 44 ++++++++++++++++++++++++----------------
>  include/hw/arm/virt.h    |  2 +-
>  3 files changed, 39 insertions(+), 24 deletions(-)
>
> diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
> index 3d1058a80c..3b6ba69a9a 100644
> --- a/docs/system/arm/virt.rst
> +++ b/docs/system/arm/virt.rst
> @@ -126,13 +126,18 @@ ras
>    Set ``on``/``off`` to enable/disable reporting host memory errors to a 
> guest
>    using ACPI and guest external abort exceptions. The default is off.
>
> +dtb-randomness
> +  Set ``on``/``off`` to pass random seeds via the guest DTB
> +  rng-seed and kaslr-seed nodes (in both "/chosen" and
> +  "/secure-chosen") to use for features like the random number
> +  generator and address space randomisation. The default is
> +  ``on``. You will want to disable it if your trusted boot chain
> +  will verify the DTB it is passed, since this option causes the
> +  DTB to be non-deterministic. It would be the responsibility of
> +  the firmware to come up with a seed and pass it on if it wants to.
> +
>  dtb-kaslr-seed
> -  Set ``on``/``off`` to pass a random seed via the guest dtb
> -  kaslr-seed node (in both "/chosen" and /secure-chosen) to use
> -  for features like address space randomisation. The default is
> -  ``on``. You will want to disable it if your trusted boot chain will
> -  verify the DTB it is passed. It would be the responsibility of the
> -  firmware to come up with a seed and pass it on if it wants to.
> +  A deprecated synonym for dtb-randomness.

We should also add a section to docs/about/deprecated.rst
noting that the old name is deprecated in favour of the new one.
I'll fold that in when I add the patch to target-arm.next, to
save you doing a respin:

--- a/docs/about/deprecated.rst
+++ b/docs/about/deprecated.rst
@@ -225,6 +225,14 @@ Use the more generic event
``DEVICE_UNPLUG_GUEST_ERROR`` instead.
 System emulator machines
 ------------------------

+Arm ``virt`` machine ``dtb-kaslr-seed`` property
+''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``dtb-kaslr-seed`` property on the ``virt`` board has been
+deprecated; use the new name ``dtb-randomness`` instead. The new name
+better reflects the way this property affects all random data within
+the device tree blob, not just the ``kaslr-seed`` node.
+

> +    object_class_property_add_bool(oc, "dtb-randomness",
> +                                   virt_get_dtb_randomness,
> +                                   virt_set_dtb_randomness);
> +    object_class_property_set_description(oc, "dtb-randomness",
> +                                          "Set off to disable passing random 
> or "
> +                                          "non-deterministic dtb nodes to 
> guest");
> +
>      object_class_property_add_bool(oc, "dtb-kaslr-seed",
> -                                   virt_get_dtb_kaslr_seed,
> -                                   virt_set_dtb_kaslr_seed);
> +                                   virt_get_dtb_randomness,
> +                                   virt_set_dtb_randomness);
>      object_class_property_set_description(oc, "dtb-kaslr-seed",
> -                                          "Set off to disable passing of 
> kaslr-seed "
> -                                          "dtb node to guest");
> +                                          "Deprecated synonym of 
> dtb-randomness");

I was going to suggest object_property_add_alias() but that
works at the object level, not the class level, so never mind.
This will work, and eventually we'll delete the code when it
hits the deprecation-to-removal horizon. So

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

and applied to target-arm.next.

thanks
-- PMM