Xen libxl (xenlight) PCI passthrough device setup: The toolstack hands a bus-mastering-capable PCI device to an HVM

Impact

The toolstack hands a bus-mastering-capable PCI device to an HVM guest before IOMMU setup for that device has completed - and when the IOMMU is disabled outright, hands it over anyway. For the duration of that window the device DMAs against raw host physical addresses with nothing in the way, so a tenant whose guest driver is ready at attach time reads or writes anywhere in host memory and escalates. This is the textbook 'DMA before the IOMMU is configured' race, and it is exactly the class of flaw that makes GPU attach/detach on a busy multi-tenant node dangerous: the risky window opens on every VM start, not once at boot.

Who can reach it

A tenant's HVM guest with any passed-through bus-mastering device, exploiting the interval between device visibility and IOMMU programming.

What to do

Apply the Xen 4.0-4.2 XSA-61 patches or move to a fixed release, and - independently of the patch - make it a hard platform invariant that passthrough is refused when the IOMMU is not active, rather than a configuration the operator can turn off. Fixing this needs a toolstack update plus a host restart of the affected domains; the durable control is verifying on every host boot that DMA remapping came up, and failing the node out of the scheduler if it did not, because a silently IOMMU-less host in a GPU fleet will otherwise keep accepting tenants.

References