Insufficient cleanup of passed-through device IRQs

ISSUE DESCRIPTION

The management of IRQs associated with physical devices exposed to x86 HVM guests involves an iterative operation in particular when cleaning up after the guest’s use of the device. In the case where an interrupt is not quiescent yet at the time this cleanup gets invoked, the cleanup attempt may be scheduled to be retried. When multiple interrupts are involved, this scheduling of a retry may get erroneously skipped. At the same time pointers may get cleared (resulting in a de-reference of NULL) and freed (resulting in a use-after-free), while other code would continue to assume them to be valid.

IMPACT

The precise impact is system specific, but would typically be a Denial of Service (DoS) affecting the entire host. Privilege escalation and information leaks cannot be ruled out.

VULNERABLE SYSTEMS

Xen versions 4.6 and later are vulnerable. Xen versions 4.5 and earlier are not vulnerable.
Only x86 HVM guests with one or more passed-through physical devices using (together) multiple physical interupts can leverage the vulnerability. x86 PV guests cannot leverage the vulnerability. x86 HVM guests without passed-through devices or with a passed-through device using just a single physical interrupt also cannot leverage the vulnerability. Device pass-through is unsupported for x86 PVH guests and all Arm guests.