Xen 4.4.x, 4.5.x, and 4.6.x does not limit the number of printk console messages when reporting unimplemented hypercalls, which allows local guests to cause a denial of service via a sequence of (1) HYPERVISOR_physdev_op hypercalls, which are not properly handled in the do_physdev_op function in arch/arm/physdev.c, or (2) HYPERVISOR_hvm_op hypercalls, which are not properly handled in the do_hvm_op function in arch/arm/hvm.c.