linux-kernel - [PATCH v2] random: treat bootloader trust toggle the same way as cpu trust toggle

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Message-Id: <20220324050930.207107-1-Jason@zx2c4.com>
Date:   Wed, 23 Mar 2022 23:09:30 -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>,
        Theodore Ts'o <tytso@....edu>,
        Graham Christensen <graham@...hamc.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: [PATCH v2] 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: Theodore Ts'o <tytso@....edu>
Cc: Graham Christensen <graham@...hamc.com>
Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
Link: https://github.com/NixOS/nixpkgs/pull/165355
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
v2 updates the documentation, using the same verbage as the trust_cpu
text.

 Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
 drivers/char/Kconfig                            | 3 ++-
 drivers/char/random.c                           | 8 +++++++-
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index c658e5d2d52c..b9d764a49207 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4400,6 +4400,12 @@
 			fully seed the kernel's CRNG. Default is controlled
 			by CONFIG_RANDOM_TRUST_CPU.
 
+	random.trust_bootloader={on,off}
+			[KNL] Enable or disable trusting the use of the
+			a seed passed by the bootloader (if available) to
+			fully seed the kernel's CRNG. Default is controlled
+			by CONFIG_RANDOM_TRUST_BOOTLOADER.
+
 	randomize_kstack_offset=
 			[KNL] Enable or disable kernel stack offset
 			randomization, which provides roughly 5 bits of
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 740811893c57..55f48375e3fe 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -449,6 +449,7 @@ config RANDOM_TRUST_BOOTLOADER
 	device randomness. Say Y here to assume the entropy provided by the
 	booloader is trustworthy so it will be added to the kernel's entropy
 	pool. Otherwise, say N here so it will be regarded as device input that
-	only mixes the entropy pool.
+	only mixes the entropy pool. This can also be configured at boot with
+	"random.trust_bootloader=on/off".
 
 endmenu
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 81dab72b611a..40107f8b9e9e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -949,11 +949,17 @@ 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
@@ -1161,7 +1167,7 @@ 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