[SRU Bionic,Focal,Focal/oem-5.6] CVE-2020-12888

[Thadeu Lima de Souza Cascardo]

[Impact]
Guests may cause DoS on the host when the latter freezes/reboots when
writing to disabled PCI devices.

[Test case]
Launch a VM with libvirt, and do a passthrough of a host PCI device (configured
using virt-manager, by adding a host device). On x86-64, the host must be
booted with intel_iommu=on on Intel machines.

Only smoke tested that passthrough still works on 4.15, as this one had the
most changes during backport. Build tested 5.4 and 5.6 kernels.

[Potential regression]
Break PCI passthrough or other uses of VFIO.