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

commit 39a8883a2b989d1d21bd8dd99f5557f0c5e89694 upstream.

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>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 88316f86cc952d7792f7b99f06d43b2168194909..d122c61ddd164a19d0b014f588a0361e523dbf8b 100644 (file)
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -590,3 +590,17 @@ source "drivers/char/xillybus/Kconfig"
 
 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 859b56d41c26361c9f000dc4434314df5d6ec19a..a18efc76fc7d5f421dcf08e714ac49ebaec07edc 100644 (file)
--- 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;
 }