AMD processors - branch predictor aliasing causing wrong branch type prediction (AMD-SB-1037): MULTI-TENANT ISOLATION

Impact

MULTI-TENANT ISOLATION: Aliases in the branch predictor cause some AMD processors to predict the wrong branch type, which is AMD's form of the RetBleed / Branch Type Confusion problem. An attacker in one security domain trains the predictor so the victim's return or branch speculates to an attacker-chosen target, leaking data across process, kernel and guest boundaries. This is the AMD analogue people look for when they ask about Spectre-BHB, and it is the real answer.

Who can reach it

Local, cross-privilege and cross-guest. Reachable from any tenant workload on affected silicon.

What to do

Needs both halves: AGESA/microcode from the OEM SBIOS package (**one to six months of lag**, drained node, power cycle) **and** an OS update that issues IBPB on context switch. Zen 1 and Zen 2 additionally depend on the LFENCE/JMP construct, which was itself found insufficient - so check that your kernel is using retpoline or IBRS rather than LFENCE/JMP by reading /sys/devices/system/cpu/vulnerabilities/spectre_v2 on the fleet. Patch alongside the other AMD-SB-1037 CVEs; they are one disclosure.

References