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>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7d1682e..6e455bc 100644 (file)
--- 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;