qemu may drive Xen into unbounded loop

ISSUE DESCRIPTION

When Xen sends requests to a device model, the next expected action inside Xen is tracked using a state field. The requests themselves are placed in a memory page shared with the device model, so that the device model can communicate to Xen its progress on the request. The state field is in the request itself, where the device model may write to it. Xen correctly rejects invalid state values, but failed to reject invalid transitions between states. As a result, a device model which switches a request between two states at the right times can drive Xen into an unbounded loop.

IMPACT

A malicious unprivileged device model can cause a Denial of Service (DoS) affecting the entire host. Specifically, it may prevent use of a physical CPU for an indeterminate period of time.

VULNERABLE SYSTEMS

All Xen versions are vulnerable.
Only x86 systems are affected. ARM systems are not affected.
Only HVM guests can expose this vulnerability. PV and PVH guests cannot expose this vulnerability, but note that the domains being able to leverage the vulnerability are PV or PVH ones, running the device model.
This vulnerability is only applicable to Xen systems using stub domains.