grant table v2 status pages may remain accessible after de-allocation (take two)

ISSUE DESCRIPTION

Guest get permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, get de-allocated when a guest switched (back) from v2 to v1. The freeing of such pages requires that the hypervisor know where in the guest these pages were mapped. The hypervisor tracks only one use within guest space, but racing requests from the guest to insert mappings of these pages may result in any of them to become mapped in multiple locations. Upon switching back from v2 to v1, the guest would then retain access to a page that was freed and perhaps re-used for other purposes.
This bug was fortuitously fixed by code cleanup in Xen 4.14, and backported to security-supported Xen branches as a prerequisite of the fix for XSA-378.

IMPACT

A malicious guest may be able to elevate its privileges to that of the host, cause host or guest Denial of Service (DoS), or cause information leaks.

VULNERABLE SYSTEMS

All Xen branches up to and including 4.13 are vulnerable, but only if the patches for XSA-378 have not been applied.
Xen versions 4.13.4, 4.14.x and 4.15.x are not affected.
Only x86 HMV and PVH guests permitted to use grant table version 2 interfaces can leverage this vulnerability. x86 PV guests cannot leverage this vulnerability. On Arm, grant table v2 use is explicitly unsupported.