linux-kernel - [PATCH 5.19 0045/1157] KVM: x86/xen: Stop Xen timer before changing IRQ

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Date:   Mon, 15 Aug 2022 19:50:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        syzbot+e54f930ed78eb0f85281@...kaller.appspotmail.com,
        Coleman Dietsch <dietschc@....edu>,
        Sean Christopherson <seanjc@...gle.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.19 0045/1157] KVM: x86/xen: Stop Xen timer before changing IRQ

From: Coleman Dietsch <dietschc@....edu>

commit c036899136355758dcd88878145036ab4d9c1f26 upstream.

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@...r.kernel.org
Link: https://syzkaller.appspot.com/bug?id=8234a9dfd3aafbf092cc5a7cd9842e3ebc45fc42
Reported-by: syzbot+e54f930ed78eb0f85281@...kaller.appspotmail.com
Signed-off-by: Coleman Dietsch <dietschc@....edu>
Reviewed-by: Sean Christopherson <seanjc@...gle.com>
Acked-by: David Woodhouse <dwmw@...zon.co.uk>
Message-Id: <20220808190607.323899-3-dietschc@....edu>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/kvm/xen.c |   33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -707,25 +707,24 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcp
 		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 (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);
+		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;
-		}
+		/* 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;