The do_block_io_op function in (1) drivers/xen/blkback/blkback.c and (2) drivers/xen/blktap/blktap.c in Xen before 3.4.0 for the Linux kernel 2.6.18, and possibly other versions, allows guest OS users to cause a denial of service (infinite loop and CPU consumption) via a large production request index to the blkback or blktap back-end drivers.  NOTE: some of these details are obtained from third party information.