arm: guest_physmap_remove_page not removing the p2m mappings

ISSUE DESCRIPTION

The functions to remove one or more entries from a guest p2m pagetable on Arm (p2m_remove_mapping, guest_physmap_remove_page, and p2m_set_entry with mfn set to INVALID_MFN) do not actually clear the pagetable entry if the entry doesn’t have the valid bit set. It is possible to have a valid pagetable entry without the valid bit set when a guest operating system uses set/way cache maintenance instructions. For instance, a guest issuing a set/way cache maintenance instruction, then calling the XENMEM_decrease_reservation hypercall to give back memory pages to Xen, might be able to retain access to those pages even after Xen started reusing them for other purposes.

IMPACT

A malicious guest may be able to access Xen and other domains’ memory. This could cause information leaks, host or domain Denial of Service (DoS), and privilege escalations.

VULNERABLE SYSTEMS

Xen version 4.12 and newer are vulnerable. Only Arm systems are vulnerable.
x86 systems are not vulnerable.