Re: [PATCH v2 1/3] efi: add support for seeding the RNG from a UEFI config table

[Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On 20 October 2016 at 12:21, Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> Specify a Linux specific UEFI configuration table that carries some
> random bits, and use the contents during early boot to seed the kernel's
> random number generator. This allows much strong random numbers to be
> generated early on.
>
> The entropy is fed to the kernel using add_device_randomness(), which is
> documented as being appropriate for being called very early.
>
> Since UEFI configuration tables may also be consumed by kexec'd kernels,
> register a reboot notifier that updates the seed in the table.
>
> Note that the config table could be generated by the EFI stub or by any
> other UEFI driver or application (e.g., GRUB), but the random seed table
> GUID and the associated functionality should be considered an internal
> kernel interface (unless it is promoted to ABI later on)
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> ---
>  drivers/firmware/efi/efi.c | 67 ++++++++++++++++++++
>  include/linux/efi.h        |  8 +++
>  2 files changed, 75 insertions(+)
>
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 1ac199cd75e7..47937ffd9f2f 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -24,6 +24,8 @@
>  #include <linux/of_fdt.h>
>  #include <linux/io.h>
>  #include <linux/platform_device.h>
> +#include <linux/random.h>
> +#include <linux/reboot.h>
>  #include <linux/slab.h>
>  #include <linux/acpi.h>
>  #include <linux/ucs2_string.h>
> @@ -48,6 +50,7 @@ struct efi __read_mostly efi = {
>         .esrt                   = EFI_INVALID_TABLE_ADDR,
>         .properties_table       = EFI_INVALID_TABLE_ADDR,
>         .mem_attr_table         = EFI_INVALID_TABLE_ADDR,
> +       .rng_seed               = EFI_INVALID_TABLE_ADDR,
>  };
>  EXPORT_SYMBOL(efi);
>
> @@ -438,6 +441,7 @@ static __initdata efi_config_table_type_t common_tables[] = {
>         {EFI_SYSTEM_RESOURCE_TABLE_GUID, "ESRT", &efi.esrt},
>         {EFI_PROPERTIES_TABLE_GUID, "PROP", &efi.properties_table},
>         {EFI_MEMORY_ATTRIBUTES_TABLE_GUID, "MEMATTR", &efi.mem_attr_table},
> +       {LINUX_EFI_RANDOM_SEED_TABLE_GUID, "RNG", &efi.rng_seed},
>         {NULL_GUID, NULL, NULL},
>  };
>
> @@ -499,6 +503,29 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
>         pr_cont("\n");
>         set_bit(EFI_CONFIG_TABLES, &efi.flags);
>
> +       if (efi.rng_seed != EFI_INVALID_TABLE_ADDR) {
> +               struct linux_efi_random_seed *seed;
> +               u32 size = 0;
> +
> +               seed = early_memremap(efi.rng_seed, sizeof(*seed));
> +               if (seed != NULL) {
> +                       size = seed->size;
> +                       early_memunmap(seed, sizeof(*seed));
> +               } else {
> +                       pr_err("Could not map UEFI random seed!\n");
> +               }
> +               if (size > 0) {
> +                       seed = early_memremap(efi.rng_seed,
> +                                             sizeof(*seed) + size);
> +                       if (seed != NULL) {
> +                               add_device_randomness(seed->bits, seed->size);
> +                               early_memunmap(seed, sizeof(*seed) + size);
> +                       } else {
> +                               pr_err("Could not map UEFI random seed!\n");
> +                       }
> +               }
> +       }
> +
>         /* Parse the EFI Properties table if it exists */
>         if (efi.properties_table != EFI_INVALID_TABLE_ADDR) {
>                 efi_properties_table_t *tbl;
> @@ -822,3 +849,43 @@ int efi_status_to_err(efi_status_t status)
>
>         return err;
>  }
> +
> +#ifdef CONFIG_KEXEC
> +static int update_efi_random_seed(struct notifier_block *nb,
> +                                 unsigned long code, void *unused)
> +{
> +       struct linux_efi_random_seed *seed;
> +       u32 size = 0;
> +

I forgot to git-add this bit here:

+       if (!kexec_in_progress)
+               return NOTIFY_DONE;
+

> +       seed = memremap(efi.rng_seed, sizeof(*seed), MEMREMAP_WB);
> +       if (seed != NULL) {
> +               size = seed->size;
> +               memunmap(seed);
> +       } else {
> +               pr_err("Could not map UEFI random seed!\n");
> +       }
> +       if (size > 0) {
> +               seed = memremap(efi.rng_seed, sizeof(*seed) + size,
> +                               MEMREMAP_WB);
> +               if (seed != NULL) {
> +                       get_random_bytes(seed->bits, seed->size);
> +                       memunmap(seed);
> +               } else {
> +                       pr_err("Could not map UEFI random seed!\n");
> +               }
> +       }
> +       return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block efi_random_seed_nb = {
> +       .notifier_call = update_efi_random_seed,
> +};
> +
> +static int register_update_efi_random_seed(void)
> +{
> +       if (efi.rng_seed == EFI_INVALID_TABLE_ADDR)
> +               return 0;
> +       return register_reboot_notifier(&efi_random_seed_nb);
> +}
> +late_initcall(register_update_efi_random_seed);
> +#endif
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 2d089487d2da..85e28b138cdd 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -599,6 +599,7 @@ void efi_native_runtime_setup(void);
>   */
>  #define LINUX_EFI_ARM_SCREEN_INFO_TABLE_GUID   EFI_GUID(0xe03fc20a, 0x85dc, 0x406e,  0xb9, 0x0e, 0x4a, 0xb5, 0x02, 0x37, 0x1d, 0x95)
>  #define LINUX_EFI_LOADER_ENTRY_GUID            EFI_GUID(0x4a67b082, 0x0a4c, 0x41cf,  0xb6, 0xc7, 0x44, 0x0b, 0x29, 0xbb, 0x8c, 0x4f)
> +#define LINUX_EFI_RANDOM_SEED_TABLE_GUID       EFI_GUID(0x1ce1e5bc, 0x7ceb, 0x42f2,  0x81, 0xe5, 0x8a, 0xad, 0xf1, 0x80, 0xf5, 0x7b)
>
>  typedef struct {
>         efi_guid_t guid;
> @@ -872,6 +873,7 @@ extern struct efi {
>         unsigned long esrt;             /* ESRT table */
>         unsigned long properties_table; /* properties table */
>         unsigned long mem_attr_table;   /* memory attributes table */
> +       unsigned long rng_seed;         /* UEFI firmware random seed */
>         efi_get_time_t *get_time;
>         efi_set_time_t *set_time;
>         efi_get_wakeup_time_t *get_wakeup_time;
> @@ -1493,4 +1495,10 @@ efi_status_t efi_exit_boot_services(efi_system_table_t *sys_table,
>                                     struct efi_boot_memmap *map,
>                                     void *priv,
>                                     efi_exit_boot_map_processing priv_func);
> +
> +struct linux_efi_random_seed {
> +       u32     size;
> +       u8      bits[];
> +};
> +
>  #endif /* _LINUX_EFI_H */
> --
> 2.7.4
>