[PATCH] efi: random: zero out secret after use and do not take minimum

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Taking the minimum is wrong, if the bootloader or EFI stub is actually
passing on a bunch of bytes that it expects the kernel to hash itself.
Ideally, a bootloader will hash it for us, but STUB won't do that, so we
should map all the bytes. Also, all those bytes must be zeroed out after
use to preserve forward secrecy.

Fixes: 161a438d730d ("efi: random: reduce seed size to 32 bytes")
Cc: stable@vger.kernel.org # v4.14+
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/firmware/efi/efi.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index f73709f7589a..819409b7b43b 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -630,7 +630,7 @@ 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 = min(seed->size, EFI_RANDOM_SEED_SIZE);
+			size = seed->size;
 			early_memunmap(seed, sizeof(*seed));
 		} else {
 			pr_err("Could not map UEFI random seed!\n");
@@ -641,6 +641,7 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
 			if (seed != NULL) {
 				pr_notice("seeding entropy pool\n");
 				add_bootloader_randomness(seed->bits, size);
+				memzero_explicit(seed->bits, size);
 				early_memunmap(seed, sizeof(*seed) + size);
 			} else {
 				pr_err("Could not map UEFI random seed!\n");
-- 
2.38.1