vsprintf: Use hw RNG for ptr_key

Currently we must wait for enough entropy to become available before
hashed pointers can be printed.  We can remove this wait by using the
hw RNG if available.

Use hw RNG to get keying material.
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

lib/vsprintf.c

@@ -1675,8 +1675,16 @@ static struct random_ready_callback random_ready = {
 
 static int __init initialize_ptr_random(void)
 {
-	int ret = add_random_ready_callback(&random_ready);
+	int key_size = sizeof(ptr_key);
+	int ret;
+
+	/* Use hw RNG if available. */
+	if (get_random_bytes_arch(&ptr_key, key_size) == key_size) {
+		static_branch_disable(&not_filled_random_ptr_key);
+		return 0;
+	}
 
+	ret = add_random_ready_callback(&random_ready);
 	if (!ret) {
 		return 0;
 	} else if (ret == -EALREADY) {