hw/arm: add control knob to disable kaslr_seed via DTB

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>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20220105135009.1584676-22-alex.bennee@linaro.org>

docs/system/arm/virt.rst

@@ -121,6 +121,14 @@ 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-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.
+
 Linux guest kernel configuration
 """"""""""""""""""""""""""""""""
 

hw/arm/virt.c

@@ -247,11 +247,15 @@ 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->dtb_kaslr_seed) {
+        create_kaslr_seed(ms, "/chosen");
+    }
 
     if (vms->secure) {
         qemu_fdt_add_subnode(fdt, "/secure-chosen");
-        create_kaslr_seed(ms, "/secure-chosen");
+        if (vms->dtb_kaslr_seed) {
+            create_kaslr_seed(ms, "/secure-chosen");
+        }
     }
 
     /* Clock node, for the benefit of the UART. The kernel device tree
@@ -2235,6 +2239,20 @@ static void virt_set_its(Object *obj, bool value, Error **errp)
     vms->its = value;
 }
 
+static bool virt_get_dtb_kaslr_seed(Object *obj, Error **errp)
+{
+    VirtMachineState *vms = VIRT_MACHINE(obj);
+
+    return vms->dtb_kaslr_seed;
+}
+
+static void virt_set_dtb_kaslr_seed(Object *obj, bool value, Error **errp)
+{
+    VirtMachineState *vms = VIRT_MACHINE(obj);
+
+    vms->dtb_kaslr_seed = value;
+}
+
 static char *virt_get_oem_id(Object *obj, Error **errp)
 {
     VirtMachineState *vms = VIRT_MACHINE(obj);
@@ -2764,6 +2782,13 @@ static void virt_machine_class_init(ObjectClass *oc, void *data)
                                           "Set on/off to enable/disable "
                                           "ITS instantiation");
 
+    object_class_property_add_bool(oc, "dtb-kaslr-seed",
+                                   virt_get_dtb_kaslr_seed,
+                                   virt_set_dtb_kaslr_seed);
+    object_class_property_set_description(oc, "dtb-kaslr-seed",
+                                          "Set off to disable passing of kaslr-seed "
+                                          "dtb node to guest");
+
     object_class_property_add_str(oc, "x-oem-id",
                                   virt_get_oem_id,
                                   virt_set_oem_id);
@@ -2828,6 +2853,9 @@ static void virt_instance_init(Object *obj)
     /* MTE is disabled by default.  */
     vms->mte = false;
 
+    /* Supply a kaslr-seed by default */
+    vms->dtb_kaslr_seed = true;
+
     vms->irqmap = a15irqmap;
 
     virt_flash_create(vms);

include/hw/arm/virt.h

@@ -148,6 +148,7 @@ struct VirtMachineState {
     bool virt;
     bool ras;
     bool mte;
+    bool dtb_kaslr_seed;
     OnOffAuto acpi;
     VirtGICType gic_version;
     VirtIOMMUType iommu;