An issue (known as XSA-212) was discovered in Xen, with fixes available for 4.8.x, 4.7.x, 4.6.x, 4.5.x, and 4.4.x. The earlier XSA-29 fix introduced an insufficient check on XENMEM_exchange input, allowing the caller to drive hypervisor memory accesses outside of the guest provided input/output arrays.
https://www.exploit-db.com/exploits/41870/
https://googleprojectzero.blogspot.com/2017/04/pandavirtualization-exploiting-xen.html
https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-029-2017.txt
http://xenbits.xen.org/xsa/advisory-212.html
http://www.securitytracker.com/id/1038223
http://www.securityfocus.com/bid/97375