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efi: random: reduce seed size to 32 bytes

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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>
This commit is contained in:
Ard Biesheuvel 2022-10-20 10:39:08 +02:00
parent f4cd18c5b2
commit 161a438d73
2 changed files with 2 additions and 2 deletions
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2
drivers/firmware/efi/efi.c
View File

@ -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));
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");

2
include/linux/efi.h
View File

@ -1222,7 +1222,7 @@ efi_status_t efi_random_get_seed(void);
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 {
u32 size;