Re: [PATCH v4] x86/speculation, KVM: remove IBPB on vCPU load

[Sean Christopherson <seanjc@google.com>]

On Thu, May 12, 2022, Sean Christopherson wrote:
> On Thu, May 12, 2022, Jon Kohler wrote:
> > Remove IBPB that is done on KVM vCPU load, as the guest-to-guest
> > attack surface is already covered by switch_mm_irqs_off() ->
> > cond_mitigation().
> > 
> > The original commit 15d45071523d ("KVM/x86: Add IBPB support") was simply
> > wrong in its guest-to-guest design intention. There are three scenarios
> > at play here:
> 
> Jim pointed offline that there's a case we didn't consider.  When switching between
> vCPUs in the same VM, an IBPB may be warranted as the tasks in the VM may be in
> different security domains.  E.g. the guest will not get a notification that vCPU0 is
> being swapped out for vCPU1 on a single pCPU.
> 
> So, sadly, after all that, I think the IBPB needs to stay.  But the documentation
> most definitely needs to be updated.
> 
> A per-VM capability to skip the IBPB may be warranted, e.g. for container-like
> use cases where a single VM is running a single workload.

Ah, actually, the IBPB can be skipped if the vCPUs have different mm_structs,
because then the IBPB is fully redundant with respect to any IBPB performed by
switch_mm_irqs_off().  Hrm, though it might need a KVM or per-VM knob, e.g. just
because the VMM doesn't want IBPB doesn't mean the guest doesn't want IBPB.

That would also sidestep the largely theoretical question of whether vCPUs from
different VMs but the same address space are in the same security domain.  It doesn't
matter, because even if they are in the same domain, KVM still needs to do IBPB.