random: If we have arch_get_random_seed*(), try it before blocking

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>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a4bea77..c35cee2 100644 (file)
--- 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;