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>

1 files changed, 33 insertions, 0 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a4bea7775e0f..c35cee268e13 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;