Re: [PATCH v6] hw/i386: pass RNG seed via setup_data entry

[Michael S. Tsirkin]

On Thu, Jul 21, 2022 at 12:49:50PM +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>

Well why not.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

who's merging this? Paolo me or you?


> ---
>  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..56896cb4b2 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"
> @@ -774,7 +775,7 @@ void x86_load_linux(X86MachineState *x86ms,
>      int dtb_size, setup_data_offset;
>      uint32_t initrd_max;
>      uint8_t header[8192], *setup, *kernel;
> -    hwaddr real_addr, prot_addr, cmdline_addr, initrd_addr = 0;
> +    hwaddr real_addr, prot_addr, cmdline_addr, initrd_addr = 0, 
> first_setup_data = 0;
>      FILE *f;
>      char *vmode;
>      MachineState *machine = MACHINE(x86ms);
> @@ -784,6 +785,7 @@ void x86_load_linux(X86MachineState *x86ms,
>      const char *dtb_filename = machine->dtb;
>      const char *kernel_cmdline = machine->kernel_cmdline;
>      SevKernelLoaderContext sev_load_ctx = {};
> +    enum { RNG_SEED_LENGTH = 32 };
>  
>      /* Align to 16 bytes as a paranoia measure */
>      cmdline_size = (strlen(kernel_cmdline) + 16) & ~15;
> @@ -1063,16 +1065,29 @@ 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 = cpu_to_le64(first_setup_data);
> +        first_setup_data = prot_addr + setup_data_offset;
>          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);
>      }
>  
> +    setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
> +    kernel_size = setup_data_offset + sizeof(struct setup_data) + 
> RNG_SEED_LENGTH;
> +    kernel = g_realloc(kernel, kernel_size);
> +    setup_data = (struct setup_data *)(kernel + setup_data_offset);
> +    setup_data->next = cpu_to_le64(first_setup_data);
> +    first_setup_data = prot_addr + setup_data_offset;
> +    setup_data->type = cpu_to_le32(SETUP_RNG_SEED);
> +    setup_data->len = cpu_to_le32(RNG_SEED_LENGTH);
> +    qemu_guest_getrandom_nofail(setup_data->data, RNG_SEED_LENGTH);
> +
> +    /* Offset 0x250 is a pointer to the first setup_data link. */
> +    stq_p(header + 0x250, first_setup_data);
> +
>      /*
>       * 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