CVE-2019-19579

An issue was discovered in Xen through 4.12.x allowing attackers to gain
host OS privileges via DMA in a situation where an untrusted domain has
access to a physical device (and assignable-add is not used), because of an
incomplete fix for CVE-2019-18424. XSA-302 relies on the use of libxl’s
“assignable-add” feature to prepare devices to be assigned to untrusted
guests. Unfortunately, this is not considered a strictly required step for
device assignment. The PCI passthrough documentation on the wiki describes
alternate ways of preparing devices for assignment, and libvirt uses its
own ways as well. Hosts where these “alternate” methods are used will still
leave the system in a vulnerable state after the device comes back from a
guest. An untrusted domain with access to a physical device can DMA into
host memory, leading to privilege escalation. Only systems where guests are
given direct access to physical devices capable of DMA (PCI pass-through)
are vulnerable. Systems which do not use PCI pass-through are not
vulnerable.