[PATCH v2 2/4] random: Use arch_get_random_seed*() at init time and once a second

H. Peter Anvin
2014-03-17T23:37:39Z

Use arch_get_random_seed*() in two places in the Linux random
driver (drivers/char/random.c):

1. During entropy pool initialization, use RDSEED in favor of RDRAND,
with a fallback to the latter. Entropy exhaustion is unlikely to
happen there on physical hardware as the machine is single-threaded
at that point, but could happen in a virtual machine. In that
case, the fallback to RDRAND will still provide more than adequate
entropy pool initialization.

2. Once a second, issue RDSEED and, if successful, feed it to the
entropy pool. To ensure an extra layer of security, only credit
half the entropy just in case.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
---
drivers/char/random.c | 24 ++++++++++++++++++++----
1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index d07575c..a4bea77 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -844,6 +844,8 @@ void add_interrupt_randomness(int irq, int irq_flags)
cycles_t cycles = random_get_entropy();
__u32 input[4], c_high, j_high;
__u64 ip;
+ unsigned long seed;
+ int credit;

c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
j_high = (sizeof(now) > 4) ? now >> 32 : 0;
@@ -862,20 +864,33 @@ void add_interrupt_randomness(int irq, int irq_flags)

r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
__mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool), NULL);
+
/*
* If we don't have a valid cycle counter, and we see
* back-to-back timer interrupts, then skip giving credit for
- * any entropy.
+ * any entropy, otherwise credit 1 bit.
*/
+ credit = 1;
if (cycles == 0) {
if (irq_flags & __IRQF_TIMER) {
if (fast_pool->last_timer_intr)
- return;
+ credit = 0;
fast_pool->last_timer_intr = 1;
} else
fast_pool->last_timer_intr = 0;
}
- credit_entropy_bits(r, 1);
+
+ /*
+ * 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.
+ */
+ if (arch_get_random_seed_long(&seed)) {
+ __mix_pool_bytes(r, &seed, sizeof(seed), NULL);
+ credit += sizeof(seed) * 4;
+ }
+
+ credit_entropy_bits(r, credit);
}

#ifdef CONFIG_BLOCK
@@ -1235,7 +1250,8 @@ static void init_std_data(struct entropy_store *r)
r->last_pulled = jiffies;
mix_pool_bytes(r, &now, sizeof(now), NULL);
for (i = r->poolinfo->poolbytes; i > 0; i -= sizeof(rv)) {
- if (!arch_get_random_long(&rv))
+ if (!arch_get_random_seed_long(&rv) &&
+ !arch_get_random_long(&rv))
rv = random_get_entropy();
mix_pool_bytes(r, &rv, sizeof(rv), NULL);
}
--
1.8.5.3