When removing or replacing a grant mapping, the x86 PV specific path needs to make sure page table entries remain in sync with other accounting done. Although the identity of the page frame was validated correctly, neither the presence of the mapping nor page writability were taken into account.
A malicious or buggy x86 PV guest could escalate its privileges or crash the hypervisor.
All Xen versions are affected. Only x86 PV guests can leverage the vulnerability. x86 HVM guests as well as ARM guests cannot leverage the vulnerability.