loop: Fix potential Spectre v1 vulnerability

type is indirectly controlled by user-space, hence leading to a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

drivers/block/loop.c:1208 loop_set_status() warn: potential spectre issue 'xfer_funcs' [r] (local cap)

Fix this by sanitizing type before using it to index xfer_funcs.

Notice that given that speculation windows are large, the policy is to kill the speculation on the first load and not worry if it can be completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

---
 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

index 0939f36548c9..015d255f451b 100644
@@ -83,6 +83,8 @@
 
 #include <linux/uaccess.h>
 
+#include <linux/nospec.h>
+
 static DEFINE_IDR(loop_index_idr);
 static DEFINE_MUTEX(loop_ctl_mutex);
 

@@ -1205,6 +1207,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 			err = -EINVAL;
 			goto out_unfreeze;
 		}
+		type = array_index_nospec(type, MAX_LO_CRYPT);
 		xfer = xfer_funcs[type];
 		if (xfer == NULL) {
 			err = -EINVAL;

2.19.2