[PATCH 3/4] iommu/vt-d: Do not enable ATS for external devices

[Mika Westerberg]

Currently Linux automatically enables ATS (Address Translation Service)
for any device that supports it (and IOMMU is turned on). ATS is used to
accelerate DMA access as the device can cache translations locally so
there is no need to do full translation on IOMMU side. However, as
pointed out in [1] ATS can be used to bypass IOMMU based security
completely by simply sending PCIe read/write transaction with AT
(Address Translation) field set to "translated".

To mitigate this modify the Intel IOMMU code so that it does not enable
ATS for any device that is marked as being external. In case this turns
out to cause performance issues we may selectively allow ATS based on
user decision but currently use big hammer and disable it completely to
be on the safe side.

[1] https://www.repository.cam.ac.uk/handle/1810/274352

Signed-off-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
---
 drivers/iommu/intel-iommu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index ada786b05a59..b79788da6971 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -1473,7 +1473,8 @@ static void iommu_enable_dev_iotlb(struct device_domain_info *info)
 	if (info->pri_supported && !pci_reset_pri(pdev) && !pci_enable_pri(pdev, 32))
 		info->pri_enabled = 1;
 #endif
-	if (info->ats_supported && !pci_enable_ats(pdev, VTD_PAGE_SHIFT)) {
+	if (!pdev->is_external && info->ats_supported &&
+	    !pci_enable_ats(pdev, VTD_PAGE_SHIFT)) {
 		info->ats_enabled = 1;
 		domain_update_iotlb(info->domain);
 		info->ats_qdep = pci_ats_queue_depth(pdev);
-- 
2.19.1