Ampere Altra and Altra Max before firmware 2.10c - PCIe root complex access control: The OS can re-initialise a PCIe

Impact

The OS can re-initialise a PCIe root complex the platform firmware deliberately disabled. Operators disable root complexes to fence off slots - a management NIC, a device belonging to another tenant partition, a port that should not exist in this SKU. Re-enabling it hands the tenant a live PCIe path to hardware they were never allocated, and a PCIe device is a DMA master. In a GPU-passthrough fleet this is the difference between 'the tenant has the two GPUs we gave them' and 'the tenant can talk to whatever else is on the fabric'.

Who can reach it

Host kernel or hypervisor code on an Altra / Altra Max node. On bare-metal rental the tenant already has this. On a virtualised Arm host it needs a prior host-kernel compromise.

What to do

Update Altra / Altra Max platform firmware to 2.10c or later via the board OEM. Flash + reboot + drain. There is no software workaround - the access control lives in firmware. Compensating control for anyone who cannot patch quickly: stop relying on 'firmware disabled that root complex' as an isolation boundary and physically depopulate or electrically isolate slots that must not be reachable.

References