x86 shadow plus log-dirty mode use-after-free

ISSUE DESCRIPTION

In environments where host assisted address translation is necessary but Hardware Assisted Paging (HAP) is unavailable, Xen will run guests in so called shadow mode. Shadow mode maintains a pool of memory used for both shadow page tables as well as auxiliary data structures. To migrate or snapshot guests, Xen additionally runs them in so called log-dirty mode. The data structures needed by the log-dirty tracking are part of aformentioned auxiliary data.
In order to keep error handling efforts within reasonable bounds, for operations which may require memory allocations shadow mode logic ensures up front that enough memory is available for the worst case requirements. Unfortunately, while page table memory is properly accounted for on the code path requiring the potential establishing of new shadows, demands by the log-dirty infrastructure were not taken into consideration. As a result, just established shadow page tables could be freed again immediately, while other code is still accessing them on the assumption that they would remain allocated.

IMPACT

Guests running in shadow mode and being subject to migration or snapshotting may be able to cause Denial of Service and other problems, including escalation of privilege.

VULNERABLE SYSTEMS

All Xen versions from at least 3.2 onwards are vulnerable. Earlier versions have not been inspected.
Only x86 systems are vulnerable. The vulnerability is limited to migration and snapshotting of guests, and only to PV ones as well as HVM or PVH ones run with shadow paging.