[PATCH] random: limit the contribution of the hw rng to at most half

Theodore Ts'o
2014-07-17T10:04:22Z

For people who don't trust a hardware RNG which can not be audited,
the changes to add support for RDSEED can be troubling since 97% or
more of the entropy will be contributed from the in-CPU hardware RNG.

We now have a in-kernel khwrngd, so for those people who do want to
implicitly trust the CPU-based system, we could create an arch-rng
hw_random driver, and allow khwrng refill the entropy pool. This
allows system administrator whether or not they trust the CPU (I
assume the NSA will trust RDRAND/RDSEED implicitly :-), and if so,
what level of entropy derating they want to use.

The reason why this is a really good idea is that if different people
use different levels of entropy derating, it will make it much more
difficult to design a backdoor'ed hwrng that can be generally
exploited in terms of the output of /dev/random when different attack
targets are using differing levels of entropy derating.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
drivers/char/random.c | 43 ++++---------------------------------------
1 file changed, 4 insertions(+), 39 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 76a56f6..e7d7ac1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -911,12 +911,13 @@ void add_interrupt_randomness(int irq, int irq_flags)

/*
* If we have architectural seed generator, produce a seed and
- * add it to the pool. For the sake of paranoia count it as
- * 50% entropic.
+ * add it to the pool. For the sake of paranoia don't let the
+ * architectural seed generator dominate the input from the
+ * interrupt noise.
*/
if (arch_get_random_seed_long(&seed)) {
__mix_pool_bytes(r, &seed, sizeof(seed));
- credit += sizeof(seed) * 4;
+ credit = 1;
}
spin_unlock(&r->lock);

@@ -1328,37 +1329,6 @@ void rand_initialize_disk(struct gendisk *disk)
}
#endif

-/*
- * Attempt an emergency refill using arch_get_random_seed_long().
- *
- * As with add_interrupt_randomness() be paranoid and only
- * credit the output as 50% entropic.
- */
-static int arch_random_refill(void)
-{
- const unsigned int nlongs = 64; /* Arbitrary number */
- unsigned int n = 0;
- unsigned int i;
- unsigned long buf[nlongs];
-
- if (!arch_has_random_seed())
- return 0;
-
- for (i = 0; i < nlongs; i++) {
- if (arch_get_random_seed_long(&buf[n]))
- n++;
- }
-
- if (n) {
- unsigned int rand_bytes = n * sizeof(unsigned long);
-
- mix_pool_bytes(&input_pool, buf, rand_bytes);
- credit_entropy_bits(&input_pool, rand_bytes*4);
- }
-
- return n;
-}
-
static ssize_t
_random_read(int nonblock, char __user *buf, size_t nbytes)
{
@@ -1379,11 +1349,6 @@ _random_read(int nonblock, char __user *buf, size_t nbytes)
return n;

/* Pool is (near) empty. Maybe wait and retry. */
-
- /* First try an emergency refill */
- if (arch_random_refill())
- continue;
-
if (nonblock)
return -EAGAIN;

--
2.0.0