Re: [PATCH v4] hw/i386: pass RNG seed via setup_data entry

[Michael S. Tsirkin]

On Thu, Jul 21, 2022 at 12:09:59PM +0200, Jason A. Donenfeld wrote:
> 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
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Richard Henderson <richard.henderson@linaro.org>
> Cc: Eduardo Habkost <eduardo@habkost.net>
> Cc: Peter Maydell <peter.maydell@linaro.org>
> Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
> Cc: Laurent Vivier <laurent@vivier.eu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  hw/i386/x86.c                                | 21 +++++++++++++++++---
>  include/standard-headers/asm-x86/bootparam.h |  1 +
>  2 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> index 6003b4b2df..284c97f158 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"
> @@ -771,7 +772,7 @@ void x86_load_linux(X86MachineState *x86ms,
>      bool linuxboot_dma_enabled = 
> X86_MACHINE_GET_CLASS(x86ms)->fwcfg_dma_enabled;
>      uint16_t protocol;
>      int setup_size, kernel_size, cmdline_size;
> -    int dtb_size, setup_data_offset;
> +    int dtb_size, setup_data_offset, last_setup_data_offset = 0;
>      uint32_t initrd_max;
>      uint8_t header[8192], *setup, *kernel;
>      hwaddr real_addr, prot_addr, cmdline_addr, initrd_addr = 0;
> @@ -1063,16 +1064,30 @@ void x86_load_linux(X86MachineState *x86ms,
>          kernel_size = setup_data_offset + sizeof(struct setup_data) + 
> dtb_size;
>          kernel = g_realloc(kernel, kernel_size);
>  
> -        stq_p(header + 0x250, prot_addr + setup_data_offset);
>          setup_data = (struct setup_data *)(kernel + setup_data_offset);
> -        setup_data->next = 0;
> +        setup_data->next = last_setup_data_offset;

does this make any difference? if the idea is that we'll add more stuff
down the road, then see below ...

>          setup_data->type = cpu_to_le32(SETUP_DTB);
>          setup_data->len = cpu_to_le32(dtb_size);
>  
>          load_image_size(dtb_filename, setup_data->data, dtb_size);
> +
> +        last_setup_data_offset = prot_addr + setup_data_offset;


if the idea is that we'll add more stuff down the road, then
it should be += here.

>      }
>  
> +    setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
> +    kernel_size = setup_data_offset + sizeof(struct setup_data) + 32;
> +    kernel = g_realloc(kernel, kernel_size);
> +    setup_data = (struct setup_data *)(kernel + setup_data_offset);
> +    setup_data->next = last_setup_data_offset;

Likely broken on LE.

> +    setup_data->type = cpu_to_le32(SETUP_RNG_SEED);
> +    setup_data->len = cpu_to_le32(32);
> +    qemu_guest_getrandom_nofail(setup_data->data, 32);
> +
> +    last_setup_data_offset = prot_addr + setup_data_offset;


where does this 32 come from? maybe make it a macro.

> +
> +    stq_p(header + 0x250, last_setup_data_offset);

add a comment while we are at it?

> +
>      /*
>       * If we're starting an encrypted VM, it will be OVMF based, which uses 
> the
>       * efi stub for booting and doesn't require any values to be placed in 
> the
> 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)
>  
> -- 
> 2.35.1