[PATCH v3 3/4] vsprintf: Use hw RNG for ptr_key

["Tobin C. Harding" <me@tobin.cc>]

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.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
---
 lib/vsprintf.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index b82f0c6c2aec..3697a19c2b25 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1657,9 +1657,8 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
 static bool have_filled_random_ptr_key __read_mostly;
 static siphash_key_t ptr_key __read_mostly;
 
-static void fill_random_ptr_key(struct random_ready_callback *unused)
+static void ptr_key_ready(void)
 {
-	get_random_bytes(&ptr_key, sizeof(ptr_key));
 	/*
 	 * have_filled_random_ptr_key==true is dependent on get_random_bytes().
 	 * ptr_to_id() needs to see have_filled_random_ptr_key==true
@@ -1669,14 +1668,28 @@ static void fill_random_ptr_key(struct random_ready_callback *unused)
 	WRITE_ONCE(have_filled_random_ptr_key, true);
 }
 
+static void fill_random_ptr_key(struct random_ready_callback *unused)
+{
+	get_random_bytes(&ptr_key, sizeof(ptr_key));
+	ptr_key_ready();
+}
+
 static struct random_ready_callback random_ready = {
 	.func = fill_random_ptr_key
 };
 
 static int __init initialize_ptr_random(void)
 {
-	int ret = add_random_ready_callback(&random_ready);
+	int ret;
+	int key_size = sizeof(ptr_key);
+
+	/* Use hw RNG if available */
+	if (get_random_bytes_arch(&ptr_key, key_size) == key_size) {
+		ptr_key_ready();
+		return 0;
+	}
 
+	ret = add_random_ready_callback(&random_ready);
 	if (!ret) {
 		return 0;
 	} else if (ret == -EALREADY) {
-- 
2.7.4