Non-maskable interrupts triggerable by guests

ISSUE DESCRIPTION

Guests are currently permitted to modify all of the (writable) bits in the PCI command register of devices passed through to them. This in particular allows them to disable memory and I/O decoding on the device unless the device is an SR-IOV virtual function, in which case subsequent accesses to the respective MMIO or I/O port ranges would

• on PCI Express devices - lead to Unsupported Request responses. The treatment of such errors is platform specific. (CVE-2015-2150)
  (Also, the patches in XSA-120 v4 and earlier were incomplete. This incompleteness is CVE-2015-8553. Additional patches are supplied in XSA-120 v5 and later to resolve this issue.)

IMPACT

In the event that the platform surfaces aforementioned UR responses as Non-Maskable Interrupts, and either the OS is configured to treat NMIs as fatal or (e.g. via ACPI’s APEI) the platform tells the OS to treat these errors as fatal, the host would crash, leading to a Denial of Service.

VULNERABLE SYSTEMS

Xen versions 3.3 and onwards are vulnerable due to supporting PCI pass-through. Upstream Linux versions 3.1 and onwards are vulnerable due to supporting PCI backend functionality. Other Linux versions as well as other OS versions may be vulnerable too.
Any domain which is given access to a non-SR-IOV virtual function PCI Express device can take advantage of this vulnerability.