On Thu, May 12, 2022 at 8:19 PM Jon Kohler <jon@xxxxxxxxxxx> wrote: > > > > > On May 12, 2022, at 11:06 PM, Jim Mattson <jmattson@xxxxxxxxxx> wrote: > > > > On Thu, May 12, 2022 at 5:50 PM Jon Kohler <jon@xxxxxxxxxxx> wrote: > > > >> You mentioned if someone was concerned about performance, are you > >> saying they also critically care about performance, such that they are > >> willing to *not* use IBPB at all, and instead just use taskset and hope > >> nothing ever gets scheduled on there, and then hope that the hypervisor > >> does the job for them? > > > > I am saying that IBPB is not the only viable mitigation for > > cross-process indirect branch steering. Proper scheduling can also > > solve the problem, without the overhead of IBPB. Say that you have two > > security domains: trusted and untrusted. If you have a two-socket > > system, and you always run trusted workloads on socket#0 and untrusted > > workloads on socket#1, IBPB is completely superfluous. However, if the > > hypervisor chooses to schedule a vCPU thread from virtual socket#0 > > after a vCPU thread from virtual socket#1 on the same logical > > processor, then it *must* execute an IBPB between those two vCPU > > threads. Otherwise, it has introduced a non-architectural > > vulnerability that the guest can't possibly be aware of. > > > > If you can't trust your OS to schedule tasks where you tell it to > > schedule them, can you really trust it to provide you with any kind of > > inter-process security? > > Fair enough, so going forward: > Should this be mandatory in all cases? How this whole effort came > was that a user could configure their KVM host with conditional > IBPB, but this particular mitigation is now always on no matter what. > > In our previous patch review threads, Sean and I mostly settled on making > this particular avenue active only when a user configures always_ibpb, such > that for cases like the one you describe (and others like it that come up in > the future) can be covered easily, but for cond_ibpb, we can document > that it doesn’t cover this case. > > Would that be acceptable here? That would make me unhappy. We use cond_ibpb, and I don't want to switch to always_ibpb, yet I do want this barrier. > > > >> Would this be the expectation of just KVM? Or all hypervisors on the > >> market? > > > > Any hypervisor that doesn't do this is broken, but that won't keep it > > off the market. :-) > > Very true :) >
