KVM: nVMX: Fix races when sending nested PI while dest enters/leaves L2

Consider the following scenario:
1. CPU A calls vmx_deliver_nested_posted_interrupt() to send an IPI
to CPU B via virtual posted-interrupt mechanism.
2. CPU B is currently executing L2 guest.
3. vmx_deliver_nested_posted_interrupt() calls
kvm_vcpu_trigger_posted_interrupt() which will note that
vcpu->mode == IN_GUEST_MODE.
4. Assume that before CPU A sends the physical POSTED_INTR_NESTED_VECTOR
IPI, CPU B exits from L2 to L0 during event-delivery
(valid IDT-vectoring-info).
5. CPU A now sends the physical IPI. The IPI is received in host and
it's handler (smp_kvm_posted_intr_nested_ipi()) does nothing.
6. Assume that before CPU A sets pi_pending=true and KVM_REQ_EVENT,
CPU B continues to run in L0 and reach vcpu_enter_guest(). As
KVM_REQ_EVENT is not set yet, vcpu_enter_guest() will continue and resume
L2 guest.
7. At this point, CPU A sets pi_pending=true and KVM_REQ_EVENT but
it's too late! CPU B already entered L2 and KVM_REQ_EVENT will only be
consumed at next L2 entry!

Another scenario to consider:
1. CPU A calls vmx_deliver_nested_posted_interrupt() to send an IPI
to CPU B via virtual posted-interrupt mechanism.
2. Assume that before CPU A calls kvm_vcpu_trigger_posted_interrupt(),
CPU B is at L0 and is about to resume into L2. Further assume that it is
in vcpu_enter_guest() after check for KVM_REQ_EVENT.
3. At this point, CPU A calls kvm_vcpu_trigger_posted_interrupt() which
will note that vcpu->mode != IN_GUEST_MODE. Therefore, do nothing and
return false. Then, will set pi_pending=true and KVM_REQ_EVENT.
4. Now CPU B continue and resumes into L2 guest without processing
the posted-interrupt until next L2 entry!

To fix both issues, we just need to change
vmx_deliver_nested_posted_interrupt() to set pi_pending=true and
KVM_REQ_EVENT before calling kvm_vcpu_trigger_posted_interrupt().

It will fix the first scenario by chaging step (6) to note that
KVM_REQ_EVENT and pi_pending=true and therefore process
nested posted-interrupt.

It will fix the second scenario by two possible ways:
1. If kvm_vcpu_trigger_posted_interrupt() is called while CPU B has changed
vcpu->mode to IN_GUEST_MODE, physical IPI will be sent and will be received
when CPU resumes into L2.
2. If kvm_vcpu_trigger_posted_interrupt() is called while CPU B hasn't yet
changed vcpu->mode to IN_GUEST_MODE, then after CPU B will change
vcpu->mode it will call kvm_request_pending() which will return true and
therefore force another round of vcpu_enter_guest() which will note that
KVM_REQ_EVENT and pi_pending=true and therefore process nested
posted-interrupt.

Cc: stable@vger.kernel.org
Fixes: 705699a1 ("KVM: nVMX: Enable nested posted interrupt processing")
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
[Add kvm_vcpu_kick to also handle the case where L1 doesn't intercept L2 HLT
 and L2 executes HLT instruction. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/vmx.c

@@ -5146,14 +5146,15 @@ static int vmx_deliver_nested_posted_interrupt(struct kvm_vcpu *vcpu,
 
 	if (is_guest_mode(vcpu) &&
 	    vector == vmx->nested.posted_intr_nv) {
-		/* the PIR and ON have been set by L1. */
-		kvm_vcpu_trigger_posted_interrupt(vcpu, true);
 		/*
 		 * If a posted intr is not recognized by hardware,
 		 * we will accomplish it in the next vmentry.
 		 */
 		vmx->nested.pi_pending = true;
 		kvm_make_request(KVM_REQ_EVENT, vcpu);
+		/* the PIR and ON have been set by L1. */
+		if (!kvm_vcpu_trigger_posted_interrupt(vcpu, true))
+			kvm_vcpu_kick(vcpu);
 		return 0;
 	}
 	return -1;