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	seed = early_memremap(efi_rng_seed, sizeof(*seed));	612	seed = early_memremap(efi_rng_seed, sizeof(*seed));
613	if (seed != NULL) {	613	if (seed != NULL) {
614	size = READ_ONCE(seed->size);	614	size = min(seed->size, EFI_RANDOM_SEED_SIZE);
615	early_memunmap(seed, sizeof(*seed));	615	early_memunmap(seed, sizeof(*seed));
616	} else {	616	} else {
617	pr_err("Could not map UEFI random seed!\n");	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	arch_efi_call_virt_teardown(); \	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	struct linux_efi_random_seed {	1227	struct linux_efi_random_seed {
1228	u32 size;	1228	u32 size;