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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 21, 2022 at 02:29:37PM +0200, Paolo Bonzini wrote:
> 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"

I am guessing we should prefix this with "x-" so we don't commit
to a user interface.

>  
>  #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