Xen on AMD-Vi - unity map handling on device reassignment: MULTI-TENANT ISOLATION: AMD-Vi unity mappings

Impact

MULTI-TENANT ISOLATION: AMD-Vi unity mappings are not correctly torn down or re-established when a device moves between guests, so mappings from a previous owner can persist. On a GPU cloud this is the device-handoff problem in its purest form: the accelerator you just reassigned to a new tenant may still carry DMA reach into the previous tenant's memory.

Who can reach it

Requires device reassignment between guests - i.e. exactly what happens when you recycle a passed-through GPU from one customer to the next.

What to do

Fixed in Xen via XSA-400. Update the hypervisor and reboot. Operationally, treat GPU reassignment as a security transition: reset the device, verify IOMMU mappings are torn down, and prefer a host reboot between tenants where your margins allow it.

References