[PATCH v3 1/3] efi: random: reduce seed size to 32 bytes

[Ard Biesheuvel <ardb@kernel.org>]

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>
---
 drivers/firmware/efi/efi.c | 2 +-
 include/linux/efi.h        | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 9624735f1575..a949509de62f 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -609,7 +609,7 @@ 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");
diff --git a/include/linux/efi.h b/include/linux/efi.h
index da3974bf05d3..cf96f8d5f15f 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1225,7 +1225,7 @@ 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;
-- 
2.35.1