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>

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.
+

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;
 }