x86/HVM pinned cache attributes mis-handling

ISSUE DESCRIPTION

To allow cachability control for HVM guests with passed through devices, an interface exists to explicitly override defaults which would otherwise be put in place. While not exposed to the affected guests themselves, the interface specifically exists for domains controlling such guests. This interface may therefore be used by not fully privileged entities, e.g. qemu running deprivileged in Dom0 or qemu running in a so called stub-domain. With this exposure it is an issue that - the number of the such controlled regions was unbounded (CVE-2022-42333), - installation and removal of such regions was not properly serialized (CVE-2022-42334).

IMPACT

Entities controlling HVM guests can run the host out of resources or stall execution of a physical CPU for effectively unbounded periods of time, resulting in a Denial of Servis (DoS) affecting the entire host. Crashes, information leaks, or elevation of privilege cannot be ruled out.

VULNERABLE SYSTEMS

Xen versions 4.11 through 4.17 are vulnerable. Older versions contain the same functionality, but it is exposed there only via an interface which is subject to XSA-77’s constraints.
Only x86 systems are potentially vulnerable. Arm systems are not vulnerable.
Only entities controlling HVM guests can leverage the vulnerability. These are device models running in either a stub domain or de-privileged in Dom0.