random: make CPU trust a boot parameter

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>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0c3c72a0ba97b79057428626c26f2d68aaa0708c..5840c7e810f55c4972bc456366b7d50b19c25275 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3526,6 +3526,12 @@
        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 d122c61ddd164a19d0b014f588a0361e523dbf8b..203d018010567836644c675b36551a7a6eba057f 100644 (file)
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -602,5 +602,5 @@ 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 15fe707cc1043f773c7340f4ecc87826b4108f23..9526bb98bb4608c217c95fc2d5e296952a90afa9 100644 (file)
--- 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;
 }