Xen qemu-xen-traditional device model hw/pt-msi.c (MSI-X passthrough): Buffer overflow on the MSI-X table write path

Impact

Buffer overflow on the MSI-X table write path for a passed-through device. A tenant with an MSI-X-capable device - which is every modern GPU and every modern NIC - writes past the end of the device model's MSI-X entry array and takes over the QEMU process. Without a device-model stub domain that process runs in dom0, so this is a direct guest-to-host escape reached through the ordinary act of a driver configuring its own interrupts. It is the most GPU-passthrough-specific escape of the pre-2018 era: the vulnerable code exists only because the device is passed through.

Who can reach it

Guest administrator - the tenant - with an assigned MSI-X-capable physical PCI device. Triggered from the guest's own device driver writing its MSI-X table.

What to do

Patch per XSA-164 and restart the affected device models, which means stopping and restarting every guest holding a passed-through device on that host - a full node drain, not a live migration, since migrating a VM with an assigned device is not generally possible. The structural mitigation worth adopting is running the device model in a stub domain so a device-model compromise lands in a deprivileged domain rather than dom0; on qemu-xen-traditional that was frequently not the default.

References