dm ioctl: prevent potential spectre v1 gadget

It appears like cmd could be a Spectre v1 gadget as it's supplied by a
user and used as an array index. Prevent the contents of kernel memory
from being leaked to userspace via speculative execution by using
array_index_nospec.
Signed-off-by: Jordy Zomer <jordy@pwning.systems>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-ioctl.c

@@ -18,6 +18,7 @@
 #include <linux/dm-ioctl.h>
 #include <linux/hdreg.h>
 #include <linux/compat.h>
+#include <linux/nospec.h>
 
 #include <linux/uaccess.h>
 #include <linux/ima.h>
@@ -1788,6 +1789,7 @@ static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags)
 	if (unlikely(cmd >= ARRAY_SIZE(_ioctls)))
 		return NULL;
 
+	cmd = array_index_nospec(cmd, ARRAY_SIZE(_ioctls));
 	*ioctl_flags = _ioctls[cmd].flags;
 	return _ioctls[cmd].fn;
 }