[PATCH v2] 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_init_region_cache() after
virtio_queue_set_num() if we are not already calling
virtio_queue_set_rings(). In the legacy path this is already done by
virtio_queue_update_rings().

Signed-off-by: Carlos López <clopez@suse.de>
---
v2: use virtio_init_region_cache() instead of
virtio_queue_update_rings() in the path for modern devices.

 hw/s390x/virtio-ccw.c      | 1 +
 hw/virtio/virtio-mmio.c    | 1 +
 hw/virtio/virtio-pci.c     | 1 +
 hw/virtio/virtio.c         | 2 +-
 include/hw/virtio/virtio.h | 1 +
 5 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index e33e5207ab..f44de1a8c1 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_init_region_cache(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..c2c6d85475 100644
--- a/hw/virtio/virtio-mmio.c
+++ b/hw/virtio/virtio-mmio.c
@@ -354,6 +354,7 @@ static void virtio_mmio_write(void *opaque, hwaddr offset, 
uint64_t value,
         if (proxy->legacy) {
             virtio_queue_update_rings(vdev, vdev->queue_sel);
         } else {
+            virtio_init_region_cache(vdev, vdev->queue_sel);
             proxy->vqs[vdev->queue_sel].num = value;
         }
         break;
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 247325c193..02fb84a8fa 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_init_region_cache(vdev, vdev->queue_sel);
         break;
     case VIRTIO_PCI_COMMON_Q_MSIX:
         vector = virtio_queue_vector(vdev, vdev->queue_sel);
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 98c4819fcc..272d930721 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -226,7 +226,7 @@ static void virtio_virtqueue_reset_region_cache(struct 
VirtQueue *vq)
     }
 }
 
-static void virtio_init_region_cache(VirtIODevice *vdev, int n)
+void virtio_init_region_cache(VirtIODevice *vdev, int n)
 {
     VirtQueue *vq = &vdev->vq[n];
     VRingMemoryRegionCaches *old = vq->vring.caches;
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 77c6c55929..fed5fff049 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -309,6 +309,7 @@ int virtio_get_num_queues(VirtIODevice *vdev);
 void virtio_queue_set_rings(VirtIODevice *vdev, int n, hwaddr desc,
                             hwaddr avail, hwaddr used);
 void virtio_queue_update_rings(VirtIODevice *vdev, int n);
+void virtio_init_region_cache(VirtIODevice *vdev, int n);
 void virtio_queue_set_align(VirtIODevice *vdev, int n, int align);
 void virtio_queue_notify(VirtIODevice *vdev, int n);
 uint16_t virtio_queue_vector(VirtIODevice *vdev, int n);
-- 
2.35.3