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. > 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 */ > 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. Thanks!