random: make CPU trust a boot parameter

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>

Documentation/admin-guide/kernel-parameters.txt

@@ -3390,6 +3390,12 @@
 	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]

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".

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;
 }