[PATCH 4.9 035/264] random: make CPU trust a boot parameter

Greg Kroah-Hartman
2022-06-23T16:48:04Z

From: Kees Cook <keescook@chromium.org>

commit 9b25436662d5fb4c66eb527ead53cab15f596ee0 upstream.

Instead of forcing a distro or other system builder to choose
at build time whether the CPU is trusted for CRNG seeding via
CONFIG_RANDOM_TRUST_CPU, provide a boot-time parameter for end users to
control the choice. The CONFIG will set the default state instead.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/kernel-parameters.txt | 6 ++++++
drivers/char/Kconfig | 4 ++--
drivers/char/random.c | 11 ++++++++---
3 files changed, 16 insertions(+), 5 deletions(-)

--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3577,6 +3577,12 @@ bytes respectively. Such letter suffixes
ramdisk_size= [RAM] Sizes of RAM disks in kilobytes
See Documentation/blockdev/ramdisk.txt.

+ random.trust_cpu={on,off}
+ [KNL] Enable or disable trusting the use of the
+ CPU's random number generator (if available) to
+ fully seed the kernel's CRNG. Default is controlled
+ by CONFIG_RANDOM_TRUST_CPU.
+
rcu_nocbs= [KNL]
The argument is a cpu list, as described above.

--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -607,5 +607,5 @@ config RANDOM_TRUST_CPU
that CPU manufacturer (perhaps with the insistence or mandate
of a Nation State's intelligence or law enforcement agencies)
has not installed a hidden back door to compromise the CPU's
- random number generation facilities.
-
+ random number generation facilities. This can also be configured
+ at boot with "random.trust_cpu=on/off".
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -780,6 +780,13 @@ static struct crng_state **crng_node_poo

static void invalidate_batched_entropy(void);

+static bool trust_cpu __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_CPU);
+static int __init parse_trust_cpu(char *arg)
+{
+ return kstrtobool(arg, &trust_cpu);
+}
+early_param("random.trust_cpu", parse_trust_cpu);
+
static void crng_initialize(struct crng_state *crng)
{
int i;
@@ -800,12 +807,10 @@ static void crng_initialize(struct crng_
}
crng->state[i] ^= rv;
}
-#ifdef CONFIG_RANDOM_TRUST_CPU
- if (arch_init) {
+ if (trust_cpu && arch_init) {
crng_init = 2;
pr_notice("random: crng done (trusting CPU's manufacturer)\n");
}
-#endif
crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
}