Xen on AMD Family 17h / Hygon Family 18h - guest SSBD selection: MULTI-TENANT ISOLATION: Setting Speculative Store

Impact

MULTI-TENANT ISOLATION: Setting Speculative Store Bypass Disable on AMD Family 17h and Hygon Family 18h has to be coordinated at the physical core level, and Xen's logic did not do that correctly. The consequence is that a guest which asked for SSBD protection may not actually get it - or may have it silently disabled by a sibling. A tenant hardening itself against Spectre-v4 gets a mitigation that is not in force, which is worse than knowing it is off.

Who can reach it

Cross-guest speculative execution between VMs sharing a physical core.

What to do

Fixed in Xen (XSA-431). Hypervisor update plus host reboot. Verify per-guest that SSBD is genuinely active afterwards rather than trusting the requested setting.

References