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 <m...@redhat.com> Cc: Marcel Apfelbaum <marcel.apfelb...@gmail.com> Cc: Paolo Bonzini <pbonz...@redhat.com> Cc: Richard Henderson <richard.hender...@linaro.org> Cc: Eduardo Habkost <edua...@habkost.net> Cc: Peter Maydell <peter.mayd...@linaro.org> Cc: Philippe Mathieu-Daudé <f4...@amsat.org> Cc: Laurent Vivier <laur...@vivier.eu> Signed-off-by: Jason A. Donenfeld <ja...@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..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
