Intel processors (return stack buffer alternate prediction): When the return stack buffer underflows, the processor

Impact

When the return stack buffer underflows, the processor falls back to an alternate predictor that can be influenced by another context, leaking information. Same structural theme as PBRSB: the return predictor is not as well isolated as the ISA implies.

Who can reach it

Local authorised code on the host.

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.

References