On Mon, 2022-02-14 at 17:14 +0000, Sean Christopherson wrote: > On Sat, Feb 12, 2022, Woodhouse, David wrote: > > (Apologies if this is HTML but I'm half-way to Austria and the laptop is > > buried somewhere in the car, and access to work email with sane email apps is > > difficult.) > > > > On 12 Feb 2022 03:05, Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > Don't actually set a request bit in vcpu->requests when making a request > > purely to force a vCPU to exit the guest. Logging the request but not > > actually consuming it causes the vCPU to get stuck in an infinite loop > > during KVM_RUN because KVM sees a pending request and bails from VM-Enter > > to service the request. > > > > > > Right, but there is no extant code which does this. The guest_uses_pa flag is > > unused. > > Grr. A WARN or something would have been nice to have. Oh well. I don't think it was clear yet what the 'wrong' behaviour would we that we should warn about, since we really hadn't finished defining the 'correct' usage :) > > The series came with a proof-of-concept that attempted using it for > > fixing nesting UAFs but it was just that — a proof of concept to demonstrate > > that the new design of GPC was sufficient to address that problem. > > > > IIRC, said proof of concept did also actually consume the req in question, > > It did. I saw that, but obviously didn't connect the dots to guest_uses_pa. > > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -9826,6 +9826,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > > if (kvm_check_request(KVM_REQ_UPDATE_CPU_DIRTY_LOGGING, vcpu)) > static_call(kvm_x86_update_cpu_dirty_logging)(vcpu); > + if (kvm_check_request(KVM_REQ_GPC_INVALIDATE, vcpu)) > + ; /* Nothing to do. It just wanted to wake us */ > Right. That's a later iteration. I originally *did* actually use it to trigger an action, IIRC, but Paolo suggested we do it differently. I also pondered having each GPC able to raise a specific request, and set the KVM_REQ_xxx bit in the GPC itself. That just made the loop in invalidate_start() a bit more complex though because of the way that it wakes the vCPUs after it's done its iteration and collected them in a cpumask. So it seemed like premature deoptimisation; we can add that in future if we really do need it, and looks like we're going in the opposite direction. > > and one of the existing test cases did exercise it with an additional mmap > > torture added? Of course until we have kernel code that *does* this, it's > > hard to exercise it from userspace :) > > Indeed. I'll send a new version with a different changelog, that way we're not > leaving a trap for developers and each architecture doesn't need to manually handle > the request. Ack, thanks.
Attachment:
smime.p7s
Description: S/MIME cryptographic signature
