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.

Ard Biesheuvel committed 4 years ago

Commit 161a4381 parent f4cd18c2 files · +2 −2Browse files

Unified · Split

2 files

drivers/firmware/efi/efi.c +1 −1
include/linux/efi.h +1 −1

drivers/firmware/efi/efi.c+1−1

@@ -611,7 +611,7 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables, · 610 unmodified lines
611	611
612	612	seed = early_memremap(efi_rng_seed, sizeof(*seed));
613	613	if (seed != NULL) {
614		− size = READ_ONCE(seed->size);
	614	+ size = min(seed->size, EFI_RANDOM_SEED_SIZE);
615	615	early_memunmap(seed, sizeof(*seed));
616	616	} else {
617	617	pr_err("Could not map UEFI random seed!\n");

include/linux/efi.h+1−1

@@ -1222,7 +1222,7 @@ efi_status_t efi_random_get_seed(void); · 1221 unmodified lines
1222	1222	arch_efi_call_virt_teardown(); \
1223	1223	})
1224	1224
1225		−#define EFI_RANDOM_SEED_SIZE 64U
	1225	+#define EFI_RANDOM_SEED_SIZE 32U // BLAKE2S_HASH_SIZE
1226	1226
1227	1227	struct linux_efi_random_seed {
1228	1228	u32 size;