pci: phantom functions assigned to incorrect contexts

ISSUE DESCRIPTION

PCI devices can make use of a functionality called phantom functions, that when enabled allows the device to generate requests using the IDs of functions that are otherwise unpopulated. This allows a device to extend the number of outstanding requests.
Such phantom functions need an IOMMU context setup, but failure to setup the context is not fatal when the device is assigned. Not failing device assignment when such failure happens can lead to the primary device being assigned to a guest, while some of the phantom functions are assigned to a different domain.

IMPACT

Under certain circumstances a malicious guest assigned a PCI device with phantom functions may be able to access memory from a previous owner of the device.

VULNERABLE SYSTEMS

Systems running all version of Xen are affected.
Only x86 systems are vulnerable. Arm systems are not vulnerable.
Only systems using PCI passthrough of devices with phantom functions are affected.