cdrom: Fix spectre-v1 gadget

This patch fixes a spectre-v1 gadget in cdrom.
The gadget could be triggered by speculatively
bypassing the cdi->capacity check.
Signed-off-by: Jordy Zomer <jordyzomer@google.com>
Link: https://lore.kernel.org/all/20230612110040.849318-2-jordyzomer@google.comReviewed-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/all/ZI1+1OG9Ut1MqsUC@equinoxSigned-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20230617113828.1230-2-phil@philpotter.co.ukSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/cdrom/cdrom.c

@@ -264,6 +264,7 @@
 #include <linux/errno.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
+#include <linux/nospec.h>
 #include <linux/slab.h> 
 #include <linux/cdrom.h>
 #include <linux/sysctl.h>
@@ -2311,6 +2312,9 @@ static int cdrom_ioctl_media_changed(struct cdrom_device_info *cdi,
 	if (arg >= cdi->capacity)
 		return -EINVAL;
 
+	/* Prevent arg from speculatively bypassing the length check */
+	barrier_nospec();
+
 	info = kmalloc(sizeof(*info), GFP_KERNEL);
 	if (!info)
 		return -ENOMEM;