[PATCH v2 3/4] random: If we have arch_get_random_seed*(), try it before blocking

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

If we have arch_get_random_seed*(), try to use it for emergency refill
of the entropy pool before giving up and blocking on /dev/random. It
may or may not work in the moment, but if it does work, it will give
the user better service than blocking will.

Reviewed-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
---
drivers/char/random.c | 33 +++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a4bea77..c35cee2 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1294,6 +1294,34 @@ 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];
+
+ 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, NULL);
+ credit_entropy_bits(&input_pool, rand_bytes*4);
+ }
+
+ return n;
+}
+
static ssize_t
random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
{
@@ -1312,8 +1340,13 @@ random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
ENTROPY_BITS(&input_pool));
if (n > 0)
return n;
+
/* Pool is (near) empty. Maybe wait and retry. */

+ /* First try an emergency refill */
+ if (arch_random_refill())
+ continue;
+
if (file->f_flags & O_NONBLOCK)
return -EAGAIN;

--
1.8.5.3