improper bug check in x86 log-dirty handling

ISSUE DESCRIPTION

Memory sharing, available to x86 HVM guests only, uses a special value in the global machine to physical address translation table (M2P). PV guests have full control over M2P entries corresponding to pages they own. A bug check (specifically, an assertion that an M2P entry is not the special “shared” indicator) was insufficiently qualified, and as a consequence is triggerable by PV guests in log-dirty mode (e.g. because of being live migrated).

IMPACT

A malicious or buggy PV guest may cause a hypervisor crash, resulting in a Denial of Service (DoS) affecting the entire host.

VULNERABLE SYSTEMS

Xen versions 4.0 and later are affected. Xen versions 3.4 and earlier are not affected.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
x86 HVM guests cannot exploit this vulnerability.
Only x86 PV guests can exploit this vulnerability, and only when being run in shadow mode. PV guests are typically run in shadow mode for live migration, as well as for features like VM snapshot.
Note that save / restore does not use shadow mode, and so does not expose this vulnerability. Some downstreams also include a “non-live migration” feature, which also does not use shadow mode (and thus does not expose this vulnerability).