Race condition in HVMOP_track_dirty_vram

ISSUE DESCRIPTION

The routine controlling the setup of dirty video RAM tracking latches the value of a pointer before taking the respective guarding lock, thus making it possible for a stale pointer to be used by the time the lock got acquired and the pointer gets dereferenced.
The hypercall providing access to the affected function is available to the domain controlling HVM guests.

IMPACT

Malicious or buggy stub domain kernels or tool stacks otherwise living outside of Domain0 can mount a denial of service attack which, if successful, can affect the whole system.
Only domains controlling HVM guests can exploit this vulnerability. (This includes domains providing hardware emulation services to HVM guests.)

VULNERABLE SYSTEMS

Xen versions from 4.0.0 onwards are vulnerable.
This vulnerability is only applicable to Xen systems using stub domains or other forms of disaggregation of control domains for HVM guests.