ZombieLoad does not affect POWER9

If it's Tuesday, there must be yet another speculative execution attack debuting with a funny name and this Tuesday's entry is ZombieLoad. ZombieLoad works on the same conceptual basis of observable speculation flaws to exfiltrate data but implements it with a new class of Intel-specific side-channel attacks utilizing a technique the investigators termed MDS, or microarchitectural data sampling. While Spectre and Meltdown attack at the cache level, ZombieLoad targets Intel HyperThreading (HT), the company's implementation of symmetric multithreading, by trying to snoop on the processor's line fill buffers (LFBs) used to load the L1 cache itself. In this case, side-channel leakages of data are possible if the malicious process triggers certain specific and ultimately invalid loads from memory -- hence the nickname -- that require microcode assistance from the CPU; these have side-effects on the LFBs which can be observed by methods similar to Spectre by other processes sharing the same CPU core. Other internal buffers of potential value can also be sussed out by related MDS-style techniques.

Because of the limited bandwidth of the LFBs and the effectively streaming nature of the technique, an attacking process can't select arbitrary addresses and therefore can't easily read arbitrary memory. Nevertheless, targeting easily recognizable kinds of data can still make the attack feasible, even against kernelspace. For example, since URLs can be picked out of memory, this apparent proof of concept shows a separate process running on the same CPU victimizing Firefox to extract the URL as the user types it in. As the user types, the values of the individual keystrokes go through the LFB to the L1 cache, allowing the malicious process to observe the changes and extract characters. By its nature there is much less data available to the attacking process but that also means there is less data to scan, making real-time attacks like this more feasible combined with other attacks or social engineering.

However, ZombieLoad is pretty much irrelevant against POWER9 because the LFBs it attempts to monitor are specific to Intel's implementation of HyperThreading (which is true for really any other SMT implementation other than Intel's; the authors of the attack say they even tried on other SMT CPUs without success, almost certainly AMD, though it is not stated for certain that they tested on Power ISA). Even for unpatched Intel machines the actual risk from this (or even most speculative execution attacks, to be sure) is probably limited because it requires running a malicious process to do the snooping and such processes almost certainly have other, more reliable ways of pwning such machines. The decision to patch may simply come down to how much risk you're willing to tolerate: nearly every Intel chip since 2011 is apparently vulnerable and the performance impact of fixing ZombieLoad varies anywhere from Intel's rosy estimate of 3-9% to up to 40% if HT must be disabled completely.