The hypercall argument translation needed for 32-bit guests running on 64-bit hypervisors performs checks on the final register state. These checks cover all registers potentially holding hypercall arguments, not just the ones actually doing so for the hypercall being processed, since the code was originally intended for use only by PV guests. A buggy or malicious HVM guest can crash the host. Acknowledgements: Red Hat would like to thank the Xen for reporting this issue.
Statement: Not vulnerable. This issue did not affect the versions of the kernel-xen package as shipped with Red Hat Enterprise Linux 5.