From 161a438d730dade2ba2b1bf8785f0759aba4ca5f Mon Sep 17 00:00:00 2001
From: Ard Biesheuvel <ardb@kernel.org>
Date: Thu, 20 Oct 2022 10:39:08 +0200
Subject: [PATCH] efi: random: reduce seed size to 32 bytes
Git-commit: 161a438d730dade2ba2b1bf8785f0759aba4ca5f
Patch-mainline: v6.1-rc4
References: git-fixes

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>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 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 3ecdc43a3f2b..a46df5d1d094 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -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");
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 80f3c1c7827d..929d559ad41d 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -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;
-- 
2.35.3