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From 5e79fc7d70e5c6e409254060066e7ab1b1d1a768 Mon Sep 17 00:00:00 2001
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Wed, 23 Mar 2022 23:09:30 -0600
Subject: [PATCH 2/7] 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@mit.edu>
Cc: Graham Christensen <graham@grahamc.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Link: https://github.com/NixOS/nixpkgs/pull/165355
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
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 59f881f36779..bdc733c2561d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4355,6 +4355,12 @@
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 3404a91edf29..19bf14e253f7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -738,11 +738,17 @@ static void invalidate_batched_entropy(void);
static void numa_crng_init(void);
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);
static bool crng_init_try_arch(struct crng_state *crng)
{
@@ -2229,7 +2235,7 @@ EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);
*/
void add_bootloader_randomness(const void *buf, unsigned int size)
{
- if (IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER))
+ if (trust_bootloader)
add_hwgenerator_randomness(buf, size, size * 8);
else
add_device_randomness(buf, size);
--
2.36.1
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