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

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>

1 files changed, 4 insertions, 39 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7d1682ea1e86..6e455bc4a39e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -910,12 +910,13 @@ 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;