HVM guest I/O port accesses are subject to either emulation or at least translation. Translations are managed by the device model (via XEN_DOMCTL_ioport_mapping), and hence the linked list used may changed at any time. Traversal of those lists (while handling guest I/O port accesses) therefore needs synchronizing with updates, which was missing so far.
Exploitability
AV:LAC:LPR:LUI:NScope
S:CImpact
C:LI:LA:H7.9/CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H