Linux x86/microcode/AMD - out-of-bounds on CPU-less NUMA nodes: The AMD microcode loader iterated every NUMA node

Impact

The AMD microcode loader iterated every NUMA node and unconditionally touched per-CPU data for each node's first CPU - which does not exist on CPU-less NUMA nodes. The result is an out-of-bounds kernel access during microcode load. CPU-less NUMA nodes are not exotic on AI hardware: they are what you get with CXL memory expanders and certain GPU/HBM topologies, so this fires on exactly the machines an AI operator runs.

Who can reach it

Local; triggered during microcode loading on systems with CPU-less NUMA nodes. Not attacker-controlled so much as a crash you hit on affected topologies.

What to do

Fixed in the Linux kernel microcode loader. Take the distro kernel update and reboot. No firmware or BIOS step. If you run CXL-attached memory or other configurations that produce memory-only NUMA nodes, treat this as a stability fix worth taking promptly rather than a security backlog item.

References