From: Jean-Philippe Brucker <[email protected]> The dtb-randomness feature, which adds random seeds to the DTB, isn't really compatible with confidential VMs since it randomizes the Realm Initial Measurement. Enabling it is not an error, but it prevents attestation. It also isn't useful to a Realm, which doesn't trust host input.
Currently the feature is automatically enabled, unless the user disables it on the command-line. Change it to OnOffAuto, and automatically disable it for confidential VMs, unless the user explicitly enables it. Signed-off-by: Jean-Philippe Brucker <[email protected]> Signed-off-by: Mathieu Poirier <[email protected]> --- docs/system/arm/virt.rst | 9 +++++---- hw/arm/virt.c | 41 +++++++++++++++++++++++++--------------- include/hw/arm/virt.h | 2 +- 3 files changed, 32 insertions(+), 20 deletions(-) diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst index 5f6dd1797862..9882b842ddd5 100644 --- a/docs/system/arm/virt.rst +++ b/docs/system/arm/virt.rst @@ -238,10 +238,11 @@ dtb-randomness 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. + ``off`` for confidential VMs, and ``on`` otherwise. 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 A deprecated synonym for dtb-randomness. diff --git a/hw/arm/virt.c b/hw/arm/virt.c index d98d8108c8d2..0f104447e008 100644 --- a/hw/arm/virt.c +++ b/hw/arm/virt.c @@ -399,6 +399,7 @@ static int gic_fdt_irq_type_spi(const VirtMachineState *vms) static void create_fdt(VirtMachineState *vms) { + bool dtb_randomness = true; MachineState *ms = MACHINE(vms); int nb_numa_nodes = ms->numa_state->num_nodes; void *fdt = create_device_tree(&vms->fdt_size); @@ -408,6 +409,16 @@ static void create_fdt(VirtMachineState *vms) exit(1); } + /* + * Including random data in the DTB causes random intial measurement on CCA, + * so disable it for confidential VMs. + */ + if (vms->dtb_randomness == ON_OFF_AUTO_OFF || + (vms->dtb_randomness == ON_OFF_AUTO_AUTO && + virt_machine_is_confidential(vms))) { + dtb_randomness = false; + } + ms->fdt = fdt; /* Header */ @@ -429,13 +440,13 @@ static void create_fdt(VirtMachineState *vms) /* /chosen must exist for load_dtb to fill in necessary properties later */ qemu_fdt_add_subnode(fdt, "/chosen"); - if (vms->dtb_randomness) { + if (dtb_randomness) { create_randomness(ms, "/chosen"); } if (vms->secure) { qemu_fdt_add_subnode(fdt, "/secure-chosen"); - if (vms->dtb_randomness) { + if (dtb_randomness) { create_randomness(ms, "/secure-chosen"); } } @@ -3510,18 +3521,21 @@ static void virt_set_virtio_transports(Object *obj, Visitor *v, vms->virtio_transports = transports; } -static bool virt_get_dtb_randomness(Object *obj, Error **errp) +static void virt_get_dtb_randomness(Object *obj, Visitor *v, const char *name, + void *opaque, Error **errp) { VirtMachineState *vms = VIRT_MACHINE(obj); + OnOffAuto dtb_randomness = vms->dtb_randomness; - return vms->dtb_randomness; + visit_type_OnOffAuto(v, name, &dtb_randomness, errp); } -static void virt_set_dtb_randomness(Object *obj, bool value, Error **errp) +static void virt_set_dtb_randomness(Object *obj, Visitor *v, const char *name, + void *opaque, Error **errp) { VirtMachineState *vms = VIRT_MACHINE(obj); - vms->dtb_randomness = value; + visit_type_OnOffAuto(v, name, &vms->dtb_randomness, errp); } static char *virt_get_oem_id(Object *obj, Error **errp) @@ -4339,16 +4353,16 @@ static void virt_machine_class_init(ObjectClass *oc, const void *data) "Set MSI settings. " "Valid values are auto, gicv2m, its and off"); - object_class_property_add_bool(oc, "dtb-randomness", - virt_get_dtb_randomness, - virt_set_dtb_randomness); + object_class_property_add(oc, "dtb-randomness", "OnOffAuto", + virt_get_dtb_randomness, virt_set_dtb_randomness, + NULL, NULL); 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_randomness, - virt_set_dtb_randomness); + object_class_property_add(oc, "dtb-kaslr-seed", "OnOffAuto", + virt_get_dtb_randomness, virt_set_dtb_randomness, + NULL, NULL); object_class_property_set_description(oc, "dtb-kaslr-seed", "Deprecated synonym of dtb-randomness"); @@ -4411,9 +4425,6 @@ static void virt_instance_init(Object *obj) /* MTE is disabled by default. */ vms->mte = false; - /* Supply kaslr-seed and rng-seed by default */ - vms->dtb_randomness = true; - vms->irqmap = a15irqmap; vms->virtio_transports = NUM_VIRTIO_TRANSPORTS; diff --git a/include/hw/arm/virt.h b/include/hw/arm/virt.h index 22e66d1a113c..d4c1aa04f664 100644 --- a/include/hw/arm/virt.h +++ b/include/hw/arm/virt.h @@ -176,7 +176,7 @@ struct VirtMachineState { bool virt; bool ras; bool mte; - bool dtb_randomness; + OnOffAuto dtb_randomness; bool second_ns_uart_present; OnOffAuto acpi; VirtGICType gic_version; -- 2.43.0
