[PATCH 4/4] hw/i386: pass RNG seed via setup_data entry

[Paolo Bonzini]

From: "Jason A. Donenfeld" <Jason@zx2c4.com>

Tiny machines optimized for fast boot time generally don't use EFI,
which means a random seed has to be supplied some other way. For this
purpose, Linux (≥5.20) supports passing a seed in the setup_data table
with SETUP_RNG_SEED, specially intended for hypervisors, kexec, and
specialized bootloaders. The linked commit shows the upstream kernel
implementation.

Link: https://git.kernel.org/tip/tip/c/68b8e9713c8
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Message-Id: <20220719115300.104095-1-Jason@zx2c4.com>
[Mostly rewritten to preserve guest ABI, but still starting from Jason's
 code. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/i386/pc.c                                 |  1 +
 hw/i386/x86.c                                | 31 ++++++++++++++++++++
 include/hw/i386/x86.h                        |  2 ++
 include/standard-headers/asm-x86/bootparam.h |  1 +
 4 files changed, 35 insertions(+)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 774cb2bf07..d456fbb166 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -112,6 +112,7 @@ const size_t pc_compat_7_0_len = 
G_N_ELEMENTS(pc_compat_7_0);
 
 GlobalProperty pc_compat_6_2[] = {
     { "virtio-mem", "unplugged-inaccessible", "off" },
+    { TYPE_X86_MACHINE, "linuxboot-seed", "off" },
 };
 const size_t pc_compat_6_2_len = G_N_ELEMENTS(pc_compat_6_2);
 
diff --git a/hw/i386/x86.c b/hw/i386/x86.c
index 564bf3834b..c5d01e084a 100644
--- a/hw/i386/x86.c
+++ b/hw/i386/x86.c
@@ -26,6 +26,7 @@
 #include "qemu/cutils.h"
 #include "qemu/units.h"
 #include "qemu/datadir.h"
+#include "qemu/guest-random.h"
 #include "qapi/error.h"
 #include "qapi/qmp/qerror.h"
 #include "qapi/qapi-visit-common.h"
@@ -1088,6 +1089,12 @@ void x86_load_linux(X86MachineState *x86ms,
     }
     fclose(f);
 
+    if (x86ms->linuxboot_seed != ON_OFF_AUTO_OFF &&
+        (data.protocol >= 0x209 || x86ms->linuxboot_seed == ON_OFF_AUTO_ON)) {
+        void *seed = add_setup_data(&data, 32, SETUP_RNG_SEED);
+        qemu_guest_getrandom_nofail(seed, 32);
+    }
+
     /* append dtb to kernel */
     if (dtb_filename) {
         dtb_size = get_image_size(dtb_filename);
@@ -1247,6 +1254,23 @@ static void x86_machine_set_smm(Object *obj, Visitor *v, 
const char *name,
     visit_type_OnOffAuto(v, name, &x86ms->smm, errp);
 }
 
+static void x86_machine_get_linuxboot_seed(Object *obj, Visitor *v, const char 
*name,
+                                           void *opaque, Error **errp)
+{
+    X86MachineState *x86ms = X86_MACHINE(obj);
+    OnOffAuto linuxboot_seed = x86ms->linuxboot_seed;
+
+    visit_type_OnOffAuto(v, name, &linuxboot_seed, errp);
+}
+
+static void x86_machine_set_linuxboot_seed(Object *obj, Visitor *v, const char 
*name,
+                                           void *opaque, Error **errp)
+{
+    X86MachineState *x86ms = X86_MACHINE(obj);
+
+    visit_type_OnOffAuto(v, name, &x86ms->linuxboot_seed, errp);
+}
+
 bool x86_machine_is_acpi_enabled(const X86MachineState *x86ms)
 {
     if (x86ms->acpi == ON_OFF_AUTO_OFF) {
@@ -1397,6 +1421,7 @@ static void x86_machine_initfn(Object *obj)
     x86ms->acpi = ON_OFF_AUTO_AUTO;
     x86ms->pit = ON_OFF_AUTO_AUTO;
     x86ms->pic = ON_OFF_AUTO_AUTO;
+    x86ms->linuxboot_seed = ON_OFF_AUTO_AUTO;
     x86ms->pci_irq_mask = ACPI_BUILD_PCI_IRQS;
     x86ms->oem_id = g_strndup(ACPI_BUILD_APPNAME6, 6);
     x86ms->oem_table_id = g_strndup(ACPI_BUILD_APPNAME8, 8);
@@ -1435,6 +1460,12 @@ static void x86_machine_class_init(ObjectClass *oc, void 
*data)
     object_class_property_set_description(oc, X86_MACHINE_PIT,
         "Enable i8254 PIT");
 
+    object_class_property_add(oc, X86_MACHINE_LINUXBOOT_SEED, "OnOffAuto",
+        x86_machine_get_linuxboot_seed, x86_machine_set_linuxboot_seed,
+        NULL, NULL);
+    object_class_property_set_description(oc, X86_MACHINE_LINUXBOOT_SEED,
+        "Pass random number seed to -kernel Linux image");
+
     object_class_property_add(oc, X86_MACHINE_PIC, "OnOffAuto",
                               x86_machine_get_pic,
                               x86_machine_set_pic,
diff --git a/include/hw/i386/x86.h b/include/hw/i386/x86.h
index 9089bdd99c..edf0f6799e 100644
--- a/include/hw/i386/x86.h
+++ b/include/hw/i386/x86.h
@@ -67,6 +67,7 @@ struct X86MachineState {
     OnOffAuto acpi;
     OnOffAuto pit;
     OnOffAuto pic;
+    OnOffAuto linuxboot_seed;
 
     char *oem_id;
     char *oem_table_id;
@@ -91,6 +92,7 @@ struct X86MachineState {
 #define X86_MACHINE_OEM_ID           "x-oem-id"
 #define X86_MACHINE_OEM_TABLE_ID     "x-oem-table-id"
 #define X86_MACHINE_BUS_LOCK_RATELIMIT  "bus-lock-ratelimit"
+#define X86_MACHINE_LINUXBOOT_SEED      "linuxboot-seed"
 
 #define TYPE_X86_MACHINE   MACHINE_TYPE_NAME("x86")
 OBJECT_DECLARE_TYPE(X86MachineState, X86MachineClass, X86_MACHINE)
diff --git a/include/standard-headers/asm-x86/bootparam.h 
b/include/standard-headers/asm-x86/bootparam.h
index 072e2ed546..b2aaad10e5 100644
--- a/include/standard-headers/asm-x86/bootparam.h
+++ b/include/standard-headers/asm-x86/bootparam.h
@@ -10,6 +10,7 @@
 #define SETUP_EFI                      4
 #define SETUP_APPLE_PROPERTIES         5
 #define SETUP_JAILHOUSE                        6
+#define SETUP_RNG_SEED                 9
 
 #define SETUP_INDIRECT                 (1<<31)
 
-- 
2.36.1