PCI/ATS: Only enable ATS for trusted devices

Add pci_ats_supported(), which checks whether a device has an ATS capability, and whether it is trusted. A device is untrusted if it is plugged into an external-facing port such as Thunderbolt and could be spoofing an existing device to exploit weaknesses in the IOMMU configuration. PCIe ATS is one such weaknesses since it allows endpoints to cache IOMMU translations and emit transactions with 'Translated' Address Type (10b) that partially bypass the IOMMU translation. The SMMUv3 and VT-d IOMMU drivers already disallow ATS and transactions with 'Translated' Address Type for untrusted devices. Add the check to pci_enable_ats() to let other drivers (AMD IOMMU for now) benefit from it. By checking ats_cap, the pci_ats_supported() helper also returns whether ATS was globally disabled with pci=noats, and could later include more things, for example whether the whole PCIe hierarchy down to the endpoint supports ATS.

JPJean-Philippe Brucker committed 6 years ago