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

Sean Christopherson
2022-04-06T21:37:49Z

On Tue, Apr 05, 2022, Sean Christopherson wrote:
> 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:

Confirmed the WARN can be triggered by abusing this patch, I'll get a patch out
once I figure out why kvm/queue is broken.

diff --git a/tools/testing/selftests/kvm/x86_64/state_test.c b/tools/testing/selftests/kvm/x86_64/state_test.c
index 2e0a92da8ff5..b7faeae3dcc4 100644
--- a/tools/testing/selftests/kvm/x86_64/state_test.c
+++ b/tools/testing/selftests/kvm/x86_64/state_test.c
@@ -210,6 +210,12 @@ int main(int argc, char *argv[])
memset(&regs1, 0, sizeof(regs1));
vcpu_regs_get(vm, VCPU_ID, &regs1);

+ if (stage == 6) {
+ state->events.flags |= 0x20;
+ vcpu_events_set(vm, VCPU_ID, &state->events);
+ vcpu_nested_state_set(vm, VCPU_ID, &state->nested, false);
+ }
+
kvm_vm_release(vm);

/* Restore state in a new VM. */