[PATCH 4.9 032/264] random: add a config option to trust the CPUs hwrng

Greg Kroah-Hartman
2022-06-23T16:47:52Z

From: Theodore Ts'o <tytso@mit.edu>

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>
---
drivers/char/Kconfig | 14 ++++++++++++++
drivers/char/random.c | 11 ++++++++++-
2 files changed, 24 insertions(+), 1 deletion(-)

--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -595,3 +595,17 @@ 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.
+
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -783,6 +783,7 @@ static void invalidate_batched_entropy(v
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);
@@ -793,10 +794,18 @@ static void crng_initialize(struct 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;
}