Commit 161a438d authored by Ard Biesheuvel's avatar Ard Biesheuvel
Browse files

efi: random: reduce seed size to 32 bytes 



We no longer need at least 64 bytes of random seed to permit the early
crng init to complete. The RNG is now based on Blake2s, so reduce the
EFI seed size to the Blake2s hash size, which is sufficient for our
purposes.

While at it, drop the READ_ONCE(), which was supposed to prevent size
from being evaluated after seed was unmapped. However, this cannot
actually happen, so READ_ONCE() is unnecessary here.

Cc: <stable@vger.kernel.org> # v4.14+
Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
Reviewed-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
Acked-by: default avatarIlias Apalodimas <ilias.apalodimas@linaro.org>
parent f4cd18c5

drivers/firmware/efi/efi.c

+1 −1
Original line number Original line Diff line number Diff line
@@ -611,7 +611,7 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables, 




		seed = early_memremap(efi_rng_seed, sizeof(*seed));

		seed = early_memremap(efi_rng_seed, sizeof(*seed));

		if (seed != NULL) {

		if (seed != NULL) {

			size = READ_ONCE(seed->size);

			size = min(seed->size, EFI_RANDOM_SEED_SIZE);

			early_memunmap(seed, sizeof(*seed));

			early_memunmap(seed, sizeof(*seed));

		} else {

		} else {

			pr_err("Could not map UEFI random seed!\n");

			pr_err("Could not map UEFI random seed!\n");

include/linux/efi.h

+1 −1
Original line number Original line Diff line number Diff line
@@ -1222,7 +1222,7 @@ efi_status_t efi_random_get_seed(void); 
	arch_efi_call_virt_teardown();					\

	arch_efi_call_virt_teardown();					\

})

})





#define EFI_RANDOM_SEED_SIZE		64U

#define EFI_RANDOM_SEED_SIZE		32U // BLAKE2S_HASH_SIZE





struct linux_efi_random_seed {

struct linux_efi_random_seed {

	u32	size;

	u32	size;