[PATCH v3 2/2] KVM: x86/xen: Stop Xen timer before changing IRQ

[Coleman Dietsch]

Stop Xen timer (if it's running) prior to changing the IRQ vector and
potentially (re)starting the timer. Changing the IRQ vector while the
timer is still running can result in KVM injecting a garbage event, e.g.
vm_xen_inject_timer_irqs() could see a non-zero xen.timer_pending from
a previous timer but inject the new xen.timer_virq.

Fixes: 536395260582 ("KVM: x86/xen: handle PV timers oneshot mode")
Cc: stable@xxxxxxxxxxxxxxx
Link: https://syzkaller.appspot.com/bug?id=8234a9dfd3aafbf092cc5a7cd9842e3ebc45fc42
Reported-by: syzbot+e54f930ed78eb0f85281@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Coleman Dietsch <dietschc@xxxxxxx>
---
 arch/x86/kvm/xen.c | 35 +++++++++++++++++------------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 6e554041e862..280cb5dc7341 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -707,26 +707,25 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		break;
 
 	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
-		if (data->u.timer.port) {
-			if (data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
-				r = -EINVAL;
-				break;
-			}
-			vcpu->arch.xen.timer_virq = data->u.timer.port;
-
-			if (!vcpu->arch.xen.timer.function)
-				kvm_xen_init_timer(vcpu);
-
-			/* Restart the timer if it's set */
-			if (data->u.timer.expires_ns)
-				kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
-						    data->u.timer.expires_ns -
-						    get_kvmclock_ns(vcpu->kvm));
-		} else if (kvm_xen_timer_enabled(vcpu)) {
-			kvm_xen_stop_timer(vcpu);
-			vcpu->arch.xen.timer_virq = 0;
+		if (data->u.timer.port &&
+		    data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
+			r = -EINVAL;
+			break;
 		}
 
+		if (!vcpu->arch.xen.timer.function)
+			kvm_xen_init_timer(vcpu);
+
+		/* Stop the timer (if it's running) before changing the vector */
+		kvm_xen_stop_timer(vcpu);
+		vcpu->arch.xen.timer_virq = data->u.timer.port;
+
+		/* Start the timer if the new value has a valid vector+expiry. */
+		if (data->u.timer.port && data->u.timer.expires_ns)
+			kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
+					    data->u.timer.expires_ns -
+					    get_kvmclock_ns(vcpu->kvm));
+
 		r = 0;
 		break;
 
-- 
2.34.1