random: add a config option to trust the CPU's hwrng

This gives the user building their own kernel (or a Linux
distribution) the option of deciding whether or not to trust the CPU's
hardware random number generator (e.g., RDRAND for x86 CPU's) as being
correctly implemented and not having a back door introduced (perhaps
courtesy of a Nation State's law enforcement or intelligence
agencies).

This will prevent getrandom(2) from blocking, if there is a
willingness to trust the CPU manufacturer.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

2 files changed, 24 insertions, 1 deletions

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 212f447938ae..ce277ee0a28a 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -554,3 +554,17 @@ config ADI
 
 endmenu
 
+config RANDOM_TRUST_CPU
+        bool "Trust the CPU manufacturer to initialize Linux's CRNG"
+        depends on X86 || S390 || PPC
+        default n
+        help
+        Assume that CPU manufacturer (e.g., Intel or AMD for RDSEED or
+        RDRAND, IBM for the S390 and Power PC architectures) is trustworthy
+        for the purposes of initializing Linux's CRNG.  Since this is not
+        something that can be independently audited, this amounts to trusting
+        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.
+
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 34ddfd57419b..f4013b8a711b 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -782,6 +782,7 @@ static void invalidate_batched_entropy(void);
 static void crng_initialize(struct crng_state *crng)
 {
         int             i;
+        int             arch_init = 1;
         unsigned long   rv;
 
         memcpy(&crng->state[0], "expand 32-byte k", 16);
@@ -792,10 +793,18 @@ static void crng_initialize(struct crng_state *crng)
                 _get_random_bytes(&crng->state[4], sizeof(__u32) * 12);
         for (i = 4; i < 16; i++) {
                 if (!arch_get_random_seed_long(&rv) &&
-                    !arch_get_random_long(&rv))
+                    !arch_get_random_long(&rv)) {
                         rv = random_get_entropy();
+                        arch_init = 0;
+                }
                 crng->state[i] ^= rv;
         }
+#ifdef CONFIG_RANDOM_TRUST_CPU
+        if (arch_init) {
+                crng_init = 2;
+                pr_notice("random: crng done (trusting CPU's manufacturer)\n");
+        }
+#endif
         crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
 }