Commit fee896d4 authored Oct 20, 2022 by Ard Biesheuvel Committed by Greg Kroah-Hartman Nov 10, 2022

efi: random: reduce seed size to 32 bytes



commit 161a438d730dade2ba2b1bf8785f0759aba4ca5f upstream.

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: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 0c727574

drivers/firmware/efi/efi.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -546,7 +546,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,

		seed = early_memremap(efi.rng_seed, sizeof(*seed));
		if (seed != NULL) {
		size = READ_ONCE(seed->size);
		size = min(seed->size, EFI_RANDOM_SEED_SIZE);
		early_memunmap(seed, sizeof(*seed));
		} else {
		pr_err("Could not map UEFI random seed!\n");

include/linux/efi.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -1715,7 +1715,7 @@ efi_status_t efi_exit_boot_services(efi_system_table_t *sys_table,
		void *priv,
		efi_exit_boot_map_processing priv_func);

		#define EFI_RANDOM_SEED_SIZE 64U
		#define EFI_RANDOM_SEED_SIZE 32U // BLAKE2S_HASH_SIZE

		struct linux_efi_random_seed {
		u32 size;