AMD Zen 1 / Zen+ / Zen 2 - L1D cache way predictor: MULTI-TENANT ISOLATION: AMD's L1D way predictor hashes virtual

Impact

MULTI-TENANT ISOLATION: AMD's L1D way predictor hashes virtual addresses to predict which cache way holds a line. Collisions in that hash are observable, giving an attacker a channel to leak metadata about a victim's memory access pattern - the researchers used it to break KASLR, to recover an AES key from a table-based implementation, and to build a covert channel between processes. It works from JavaScript in a browser and across VMs, which is unusually broad reach for a microarchitectural channel.

Who can reach it

Local, unprivileged, co-resident with the victim on the same physical core. Affects Zen 1, Zen+ and Zen 2 (2017-2019 EPYC generations).

What to do

**No CVE was assigned and AMD issued no microcode fix**, taking the position that existing side-channel guidance and secret-independent software already cover it. Treat this as unpatchable on affected silicon. Operator-side controls: do not co-schedule tenants on the same physical core, disable SMT on mixed-tenancy nodes, and prefer newer EPYC generations for workloads where cross-tenant leakage is in your threat model. Nothing here requires a reboot or firmware - it is a scheduling and fleet-composition decision.

References