random: make CPU trust a boot parameter

kees · tytso · commit 9b25436662d5 · 2018-09-01T12:51:54.000-04:00
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 &lt;keescook@chromium.org&gt;
Signed-off-by: Theodore Ts'o &lt;tytso@mit.edu&gt;
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/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]
diff --git 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
@@ -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;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -779,6 +779,13 @@ static struct crng_state **crng_node_pool __read_mostly;`
`779`	`779`
`780`	`780`	`static void invalidate_batched_entropy(void);`
`781`	`781`
	`782`	`+static bool trust_cpu __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_CPU);`
	`783`	`+static int __init parse_trust_cpu(char *arg)`
	`784`	`+{`
	`785`	`+ return kstrtobool(arg, &trust_cpu);`
	`786`	`+}`
	`787`	`+early_param("random.trust_cpu", parse_trust_cpu);`
	`788`	`+`
`782`	`789`	`static void crng_initialize(struct crng_state *crng)`
`783`	`790`	`{`
`784`	`791`	`int i;`
`@@ -799,12 +806,10 @@ static void crng_initialize(struct crng_state *crng)`
`799`	`806`	`}`
`800`	`807`	`crng->state[i] ^= rv;`
`801`	`808`	`}`
`802`		`-#ifdef CONFIG_RANDOM_TRUST_CPU`
`803`		`- if (arch_init) {`
	`809`	`+ if (trust_cpu && arch_init) {`
`804`	`810`	`crng_init = 2;`
`805`	`811`	`pr_notice("random: crng done (trusting CPU's manufacturer)\n");`
`806`	`812`	`}`
`807`		`-#endif`
`808`	`813`	`crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;`
`809`	`814`	`}`
`810`	`815`