MMU update operations targeting page tables are intended to be used on PV guests only. The lack of a respective check made it possible for such operations to access certain function pointers which remain NULL when the target guest is using Hardware Assisted Paging (HAP).
Malicious or buggy stub domain kernels or tool stacks otherwise living outside of Domain0 can mount a denial of service or privilege escalation attack which, if successful, can affect the whole system. Only PV domains with privilege over other guests can exploit this vulnerability; and only when those other guests are HVM using HAP, or PVH. The vulnerability is therefore exposed to PV domains providing hardware emulation services to HVM guests.
Xen 4.0 and onward are vulnerable. Only x86 systems are vulnerable. ARM systems are not vulnerable. The vulnerability is only exposed to PV service domains for HVM or PVH guests which have privilege over the guest. In a usual configuration that means only device model emulators (qemu-dm). In the case of HVM guests whose device model is running in an unrestricted dom0 process, qemu-dm already has the ability to cause problems for the whole system. So in that case the vulnerability is not applicable. The situation is more subtle for an HVM guest with a stub qemu-dm. That is, where the device model runs in a separate domain (in the case of xl, as requested by "device_model_stubdomain_override=1" in the xl domain configuration file). The same applies with a qemu-dm in a dom0 process subjected to some kind kernel-based process privilege limitation (eg the chroot technique as found in some versions of XCP/XenServer). In those latter situations this issue means that the extra isolation does not provide as good a defence as intended. That is the essence of this vulnerability. However, the security is still better than with a qemu-dm running as an unrestricted dom0 process. Therefore users with these configurations should not switch to an unrestricted dom0 qemu-dm. Finally, in a radically disaggregated system: where the HVM or PVH service domain software (probably, the device model domain image in the HVM case) is not always supplied by the host administrator, a malicious service domain administrator can exercise this vulnerability.