diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index 5294abb3f178..8ffd07e2a160 100644
--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -40,7 +40,7 @@ static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev,
 	return 1;
 }
 
-static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev)
+static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev, u32 last)
 {
 	struct kvm_coalesced_mmio_ring *ring;
 	unsigned avail;
@@ -52,7 +52,7 @@ static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev)
 	 * there is always one unused entry in the buffer
 	 */
 	ring = dev->kvm->coalesced_mmio_ring;
-	avail = (ring->first - ring->last - 1) % KVM_COALESCED_MMIO_MAX;
+	avail = (ring->first - last - 1) % KVM_COALESCED_MMIO_MAX;
 	if (avail == 0) {
 		/* full */
 		return 0;
@@ -67,25 +67,28 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
 {
 	struct kvm_coalesced_mmio_dev *dev = to_mmio(this);
 	struct kvm_coalesced_mmio_ring *ring = dev->kvm->coalesced_mmio_ring;
+	__u32 insert;
 
 	if (!coalesced_mmio_in_range(dev, addr, len))
 		return -EOPNOTSUPP;
 
 	spin_lock(&dev->kvm->ring_lock);
 
-	if (!coalesced_mmio_has_room(dev)) {
+	insert = READ_ONCE(ring->last);
+	if (!coalesced_mmio_has_room(dev, insert) ||
+	    insert >= KVM_COALESCED_MMIO_MAX) {
 		spin_unlock(&dev->kvm->ring_lock);
 		return -EOPNOTSUPP;
 	}
 
 	/* copy data in first free entry of the ring */
 
-	ring->coalesced_mmio[ring->last].phys_addr = addr;
-	ring->coalesced_mmio[ring->last].len = len;
-	memcpy(ring->coalesced_mmio[ring->last].data, val, len);
-	ring->coalesced_mmio[ring->last].pio = dev->zone.pio;
+	ring->coalesced_mmio[insert].phys_addr = addr;
+	ring->coalesced_mmio[insert].len = len;
+	memcpy(ring->coalesced_mmio[insert].data, val, len);
+	ring->coalesced_mmio[insert].pio = dev->zone.pio;
 	smp_wmb();
-	ring->last = (ring->last + 1) % KVM_COALESCED_MMIO_MAX;
+	ring->last = (insert + 1) % KVM_COALESCED_MMIO_MAX;
 	spin_unlock(&dev->kvm->ring_lock);
 	return 0;
 }