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@xxxxxxxxxxxx>
---
Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
drivers/char/Kconfig | 4 ++--
drivers/char/random.c | 11 ++++++++---
3 files changed, 16 insertions(+), 5 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9871e649ffef..64a3bf54b974 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3523,6 +3523,12 @@
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.
+
ras=option[,option,...] [KNL] RAS-specific options
cec_disable [X86]
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index ce277ee0a28a..40728491f37b 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -566,5 +566,5 @@ 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".
diff --git a/drivers/char/random.c b/drivers/char/random.c
index bf5f99fc36f1..c75b6cdf0053 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -779,6 +779,13 @@ static struct crng_state **crng_node_pool __read_mostly;
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;
@@ -799,12 +806,10 @@ static void crng_initialize(struct crng_state *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;
}
--
2.17.1
--
Kees Cook
Pixel Security
