Re: [PATCH] hw/arm: add control knob to disable kaslr_seed via DTB

[Ilias Apalodimas]

Hi Alex,

On Wed, 15 Dec 2021 at 14:09, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> Generally a guest needs an external source of randomness to properly
> enable things like address space randomisation. However in a trusted
> boot environment where the firmware will cryptographically verify
> components having random data in the DTB will cause verification to
> fail. Add a control knob so we can prevent this being added to the
> system DTB.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> Cc: Jerome Forissier <jerome@forissier.org>
> ---
>  docs/system/arm/virt.rst |  7 +++++++
>  include/hw/arm/virt.h    |  1 +
>  hw/arm/virt.c            | 32 ++++++++++++++++++++++++++++++--
>  3 files changed, 38 insertions(+), 2 deletions(-)
>
> diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
> index 850787495b..c86a4808df 100644
> --- a/docs/system/arm/virt.rst
> +++ b/docs/system/arm/virt.rst
> @@ -121,6 +121,13 @@ 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.
>
> +kaslr-dtb-seed
> +  Set ``on``/``off`` to pass a random seed via the guest dtb 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.
> +
>  Linux guest kernel configuration
>  """"""""""""""""""""""""""""""""
>
> diff --git a/include/hw/arm/virt.h b/include/hw/arm/virt.h
> index dc6b66ffc8..acd0665fe7 100644
> --- a/include/hw/arm/virt.h
> +++ b/include/hw/arm/virt.h
> @@ -148,6 +148,7 @@ struct VirtMachineState {
>      bool virt;
>      bool ras;
>      bool mte;
> +    bool kaslr_dtb_seed;
>      OnOffAuto acpi;
>      VirtGICType gic_version;
>      VirtIOMMUType iommu;
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 30da05dfe0..4496612e89 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -248,11 +248,15 @@ static void create_fdt(VirtMachineState *vms)
>
>      /* /chosen must exist for load_dtb to fill in necessary properties later 
> */
>      qemu_fdt_add_subnode(fdt, "/chosen");
> -    create_kaslr_seed(ms, "/chosen");
> +    if (vms->kaslr_dtb_seed) {
> +        create_kaslr_seed(ms, "/chosen");
> +    }
>
>      if (vms->secure) {
>          qemu_fdt_add_subnode(fdt, "/secure-chosen");
> -        create_kaslr_seed(ms, "/secure-chosen");
> +        if (vms->kaslr_dtb_seed) {
> +            create_kaslr_seed(ms, "/secure-chosen");
> +        }
>      }
>
>      /* Clock node, for the benefit of the UART. The kernel device tree
> @@ -2236,6 +2240,20 @@ static void virt_set_its(Object *obj, bool value, 
> Error **errp)
>      vms->its = value;
>  }
>
> +static bool virt_get_kaslr_dtb_seed(Object *obj, Error **errp)
> +{
> +    VirtMachineState *vms = VIRT_MACHINE(obj);
> +
> +    return vms->kaslr_dtb_seed;
> +}
> +
> +static void virt_set_kaslr_dtb_seed(Object *obj, bool value, Error **errp)
> +{
> +    VirtMachineState *vms = VIRT_MACHINE(obj);
> +
> +    vms->kaslr_dtb_seed = value;
> +}
> +
>  static char *virt_get_oem_id(Object *obj, Error **errp)
>  {
>      VirtMachineState *vms = VIRT_MACHINE(obj);
> @@ -2765,6 +2783,13 @@ static void virt_machine_class_init(ObjectClass *oc, 
> void *data)
>                                            "Set on/off to enable/disable "
>                                            "ITS instantiation");
>
> +    object_class_property_add_bool(oc, "kaslr-dtb-seed",
> +                                   virt_get_kaslr_dtb_seed,
> +                                   virt_set_kaslr_dtb_seed);
> +    object_class_property_set_description(oc, "kaslr-dtb-seed",
> +                                          "Set off to disable passing of 
> kaslr "
> +                                          "dtb node to guest");
> +
>      object_class_property_add_str(oc, "x-oem-id",
>                                    virt_get_oem_id,
>                                    virt_set_oem_id);
> @@ -2829,6 +2854,9 @@ static void virt_instance_init(Object *obj)
>      /* MTE is disabled by default.  */
>      vms->mte = false;
>
> +    /* Supply a kaslr-seed by default */
> +    vms->kaslr_dtb_seed = true;
> +
>      vms->irqmap = a15irqmap;
>
>      virt_flash_create(vms);
> --
> 2.30.2
>

This is particularly useful if the bootloader uses a TPM to measures
the DTB and end up with a measured boot flow.

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>