Re: [PATCH v5 1/3] KVM: X86: Save&restore the triple fault request

Sean Christopherson
2022-04-06T05:20:42Z

On Tue, Apr 05, 2022, Sean Christopherson wrote:
> On Fri, Mar 18, 2022, Chenyi Qiang wrote:
> > @@ -4976,6 +4980,9 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
> > }
> > }
> >
> > + if (events->flags & KVM_VCPUEVENT_TRIPLE_FAULT)
> > + kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> > +
> > kvm_make_request(KVM_REQ_EVENT, vcpu);
>
> Looks correct, but this really needs a selftest, at least for the SET path since
> the intent is to use that for the NOTIFY handling. Doesn't need to be super fancy,
> e.g. do port I/O from L2, inject a triple fault, and verify L1 sees the appropriate
> exit.
>
> Aha! And for the GET path, abuse KVM_X86_SET_MCE with CR4.MCE=0 to coerce KVM into
> making a KVM_REQ_TRIPLE_FAULT, that way there's no need to try and hit a timing
> window to intercept the request.

Drat, I bet that MCE path means the WARN in nested_vmx_vmexit() can be triggered
by userspace. If so, this patch makes it really, really easy to hit, e.g. queue the
request while L2 is active, then do KVM_SET_NESTED_STATE to force an "exit" without
bouncing through kvm_check_nested_events().

WARN_ON_ONCE(kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu))

I don't think SVM has a user-triggerable WARN, but the request should still be
dropped on forced exit from L2, e.g. I believe this is the correct fix:

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 9a6dc2b38fcf..18c5e96b12a5 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1128,6 +1128,7 @@ void svm_leave_nested(struct kvm_vcpu *vcpu)
if (is_guest_mode(vcpu)) {
svm->nested.nested_run_pending = 0;
svm->nested.vmcb12_gpa = INVALID_GPA;
+ kvm_clear_request(KVM_REQ_TRIPLE_FAULT, vcpu);

leave_guest_mode(vcpu);

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index f18744f7ff82..0587ef647553 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6276,6 +6276,7 @@ void vmx_leave_nested(struct kvm_vcpu *vcpu)
{
if (is_guest_mode(vcpu)) {
to_vmx(vcpu)->nested.nested_run_pending = 0;
+ kvm_clear_request(KVM_REQ_TRIPLE_FAULT, vcpu);
nested_vmx_vmexit(vcpu, -1, 0, 0);
}
free_nested(vcpu);