[PATCH 4.14 29/40] efi: random: reduce seed size to 32 bytes

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Ard Biesheuvel <ardb@kernel.org>

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

--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -532,7 +532,7 @@ int __init efi_config_parse_tables(void
 
 		seed = early_memremap(efi.rng_seed, sizeof(*seed));
 		if (seed != NULL) {
-			size = 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");
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1607,7 +1607,7 @@ efi_status_t efi_exit_boot_services(efi_
 				    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;