linux-kernel - [PATCH] random: treat bootloader trust toggle the same way as cpu trust toggle

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Date:   Tue, 22 Mar 2022 22:11:23 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Theodore Ts'o <tytso@....edu>,
        Graham Christensen <graham@...hamc.com>
Subject: [PATCH] random: treat bootloader trust toggle the same way as cpu trust toggle

If CONFIG_RANDOM_TRUST_CPU is set, the RNG initializes using RDRAND.
But, the user can disable (or enable) this behavior by setting
`random.trust_cpu=0/1` on the kernel command line. This allows system
builders to do reasonable things while avoiding howls from tinfoil
hatters. (Or vice versa.)

CONFIG_RANDOM_TRUST_BOOTLOADER is basically the same thing, but regards
the seed passed via EFI or device tree, which might come from RDRAND or
a TPM or somewhere else. In order to allow distros to more easily enable
this while avoiding those same howls (or vice versa), this commit adds
the corresponding `random.trust_bootloader=0/1` toggle.

Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Theodore Ts'o <tytso@....edu>
Cc: Graham Christensen <graham@...hamc.com>
Link: https://github.com/NixOS/nixpkgs/pull/165355
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7b7f5e6596c2..c8974e5f57e1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -948,11 +948,17 @@ static bool drain_entropy(void *buf, size_t nbytes, bool force)
  **********************************************************************/
 
 static bool trust_cpu __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_CPU);
+static bool trust_bootloader __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER);
 static int __init parse_trust_cpu(char *arg)
 {
 	return kstrtobool(arg, &trust_cpu);
 }
+static int __init parse_trust_bootloader(char *arg)
+{
+	return kstrtobool(arg, &trust_bootloader);
+}
 early_param("random.trust_cpu", parse_trust_cpu);
+early_param("random.trust_bootloader", parse_trust_bootloader);
 
 /*
  * The first collection of entropy occurs at system boot while interrupts
@@ -1160,7 +1166,7 @@ EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);
  */
 void add_bootloader_randomness(const void *buf, size_t size)
 {
-	if (IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER))
+	if (trust_bootloader)
 		add_hwgenerator_randomness(buf, size, size * 8);
 	else
 		add_device_randomness(buf, size);
-- 
2.35.1