linux-kernel - [RFC] random: UEFI RNG input is bootloader randomness

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Date:   Sat, 28 Sep 2019 12:14:28 +0200
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     ard.biesheuvel@...aro.org, hsinyi@...omium.org,
        swboyd@...omium.org, robh@...nel.org, tytso@....edu,
        keescook@...omium.org, joeyli.kernel@...il.com
Cc:     linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org
Subject: [RFC] random: UEFI RNG input is bootloader randomness

Depending on RANDOM_TRUST_BOOTLOADER, bootloader-provided randomness
is credited as entropy. As the UEFI seeding entropy pool is seeded by
the UEFI firmware/bootloader, add its content as bootloader randomness.

Note that this UEFI (v2.4 or newer) feature is currently only
implemented for EFI stub booting on ARM, and further note that
RANDOM_TRUST_BOOTLOADER must only be enabled if there indeed is
sufficient trust in the bootloader _and_ its source of randomness.

Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: Hsin-Yi Wang <hsinyi@...omium.org>
Cc: Stephen Boyd <swboyd@...omium.org>
Cc: Rob Herring <robh@...nel.org>
Cc: Theodore Ts'o <tytso@....edu>
Cc: Lee, Chun-Yi <joeyli.kernel@...il.com>

---

Untested patch, as efi_random_get_seed() is only hooked up on ARM,
and the firmware on my old x86 laptop only has UEFI v2.31 anyway.

Thanks,
	Dominik

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 8f1ab04f6743..db0bffce754e 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -545,7 +545,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
 					      sizeof(*seed) + size);
 			if (seed != NULL) {
 				pr_notice("seeding entropy pool\n");
-				add_device_randomness(seed->bits, seed->size);
+				add_bootloader_randomness(seed->bits, seed->size);
 				early_memunmap(seed, sizeof(*seed) + size);
 			} else {
 				pr_err("Could not map UEFI random seed!\n");