The XENMEM_exchange handler in Xen 4.2 and earlier does not properly check the memory address, which allows local PV guest OS administrators to cause a denial of service (crash) or possibly gain privileges via unspecified vectors that overwrite memory in the hypervisor reserved range.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.