Intel Core Ultra processors (branch prediction unit initialisation): MULTI-TENANT ISOLATION: Part of the Training Solo

Impact

MULTI-TENANT ISOLATION: Part of the Training Solo family: incorrect initialisation of the branch prediction unit lets an attacker self-train a predictor within the victim's own domain, so the leak works without the cross-domain training that existing mitigations assume. That is the significance - it sidesteps domain-isolation mitigations rather than defeating them head-on, and it reopens guest-to-host and user-to-kernel leakage on parts believed fixed.

Who can reach it

Local unprivileged code on an affected processor.

What to do

Mitigated by an Intel microcode update plus OS/hypervisor changes. Microcode for this class is normally shipped by your distribution as an early-loadable image, so you can deploy it with a package update and a reboot without waiting for an OEM BIOS release - that distinction is the difference between a week and a quarter. Verify after reboot by reading /sys/devices/system/cpu/vulnerabilities/ rather than assuming the package took effect. Training Solo also needs kernel-side changes for the eBPF and indirect-branch paths; take both.

References