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

[Greg Kroah-Hartman]

From: Kees Cook <keescook@xxxxxxxxxxxx>

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@xxxxxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 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;
 }