The sg_build_indirect function in drivers/scsi/sg.c in Linux kernel 2.6.28-rc1 through 2.6.31-rc8 uses an incorrect variable when accessing an array, which allows local users to cause a denial of service (kernel OOPS and NULL pointer dereference), as demonstrated by using xcdroast to duplicate a CD.  NOTE: this is only exploitable by users who can open the cdrom device.