xen crash due to use after free on hvm guest teardown

ISSUE DESCRIPTION

Certain data accessible (via hypercalls) by the domain controlling the execution of a HVM domain is being freed prematurely, leading to the respective memory regions to possibly be read from and written to in ways unexpected by their new owner(s).

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 4.2 onwards are vulnerable on x86 systems. ARM systems are not vulnerable.
This vulnerability is only applicable to Xen systems using stub domains or other forms of disaggregation of control domains for HVM guests.