QEMU, as used in Xen 3.3.x through 4.5.x, does not properly restrict access to PCI command registers, which might allow local HVM guest users to cause a denial of service (non-maskable interrupt and host crash) by disabling the (1) memory or (2) I/O decoding for a PCI Express device and then accessing the device, which triggers an Unsupported Request (UR) response.
Exploitability
AV:LAC:LAu:NImpact
C:NI:NA:C4.9/AV:L/AC:L/Au:N/C:N/I:N/A:C