Re: [PATCH 1/2] efi: add support for seeding the RNG from a UEFI config table

[Mark Rutland <mark.rutland@arm.com>]

Hi Ard,

On Thu, Oct 06, 2016 at 11:27:25AM +0100, Ard Biesheuvel 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.
> 
> 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)

What does this mean for kexec? Won't each successive kernel look for the
table and find the same seed?

I think to some extent this mush be treated as an ABI, given cases like
kexec.

Thanks,
Mark.

> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  drivers/firmware/efi/efi.c | 26 ++++++++++++++++++++
>  include/linux/efi.h        |  8 ++++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 1ac199cd75e7..c8ae40f9b674 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -24,6 +24,7 @@
>  #include <linux/of_fdt.h>
>  #include <linux/io.h>
>  #include <linux/platform_device.h>
> +#include <linux/random.h>
>  #include <linux/slab.h>
>  #include <linux/acpi.h>
>  #include <linux/ucs2_string.h>
> @@ -48,6 +49,7 @@ 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 +440,7 @@ 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 +502,29 @@ 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;
> 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
> 
> --
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