xen: denial of service

Internal data of the emulator for MMIO operations may, under certain rare conditions, at the end of one emulation cycle be left in a state affecting a subsequent emulation such that this second emulation would fail, causing an exception to be reported to the guest kernel where none is expected.

Guest mode unprivileged (user) code, which has been granted the privilege to access MMIO regions, may leverage that access to crash the whole guest.

Only HVM guests exposing MMIO ranges to unprivileged (user) mode are vulnerable to this issue. PV guests are not.