[PATCH] virtio: refresh vring region cache after updating a virtqueue size

[Carlos López]

When a virtqueue size is changed by the guest via
virtio_queue_set_num(), its region cache is not automatically updated.
If the size was increased, this could lead to accessing the cache out
of bounds. For example, in vring_get_used_event():

    static inline uint16_t vring_get_used_event(VirtQueue *vq)
    {
        return vring_avail_ring(vq, vq->vring.num);
    }

    static inline uint16_t vring_avail_ring(VirtQueue *vq, int i)
    {
        VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
        hwaddr pa = offsetof(VRingAvail, ring[i]);

        if (!caches) {
            return 0;
        }

        return virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
    }

vq->vring.num will be greater than caches->avail.len, which will
trigger a failed assertion down the call path of
virtio_lduw_phys_cached().

Fix this by calling virtio_queue_update_rings() after
virtio_queue_set_num() if we are not already calling
virtio_queue_set_rings().

Signed-off-by: Carlos López <clopez@suse.de>
---
 hw/s390x/virtio-ccw.c   | 1 +
 hw/virtio/virtio-mmio.c | 5 ++---
 hw/virtio/virtio-pci.c  | 1 +
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index e33e5207ab..89891ac58a 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -237,6 +237,7 @@ static int virtio_ccw_set_vqs(SubchDev *sch, VqInfoBlock 
*info,
                 return -EINVAL;
             }
             virtio_queue_set_num(vdev, index, num);
+            virtio_queue_update_rings(vdev, index);
         } else if (virtio_queue_get_num(vdev, index) > num) {
             /* Fail if we don't have a big enough queue. */
             return -EINVAL;
diff --git a/hw/virtio/virtio-mmio.c b/hw/virtio/virtio-mmio.c
index 23ba625eb6..c74822308f 100644
--- a/hw/virtio/virtio-mmio.c
+++ b/hw/virtio/virtio-mmio.c
@@ -350,10 +350,9 @@ static void virtio_mmio_write(void *opaque, hwaddr offset, 
uint64_t value,
     case VIRTIO_MMIO_QUEUE_NUM:
         trace_virtio_mmio_queue_write(value, VIRTQUEUE_MAX_SIZE);
         virtio_queue_set_num(vdev, vdev->queue_sel, value);
+        virtio_queue_update_rings(vdev, vdev->queue_sel);
 
-        if (proxy->legacy) {
-            virtio_queue_update_rings(vdev, vdev->queue_sel);
-        } else {
+        if (!proxy->legacy) {
             proxy->vqs[vdev->queue_sel].num = value;
         }
         break;
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 247325c193..a0a2f2c965 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -1554,6 +1554,7 @@ static void virtio_pci_common_write(void *opaque, hwaddr 
addr,
         proxy->vqs[vdev->queue_sel].num = val;
         virtio_queue_set_num(vdev, vdev->queue_sel,
                              proxy->vqs[vdev->queue_sel].num);
+        virtio_queue_update_rings(vdev, vdev->queue_sel);
         break;
     case VIRTIO_PCI_COMMON_Q_MSIX:
         vector = virtio_queue_vector(vdev, vdev->queue_sel);
-- 
2.35.3