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>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0c8f7889efa1f9122bbfeaf5b2369090e6dfa093..227c5c6fa4c1792d36bcda67775111a8e358449f 100644 (file)
--- 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
index ce277ee0a28a21a328bee687d6872842a1cda5cc..40728491f37b6a46be7e59399c6530263d2d40c1 100644 (file)
--- 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 bf5f99fc36f1b654aad2f4cfe196c9b66814be82..c75b6cdf00533ac82ec6fb957bc1669f1f44a34d 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;
 }