virtio: fix off-by-one and invalid access in virtqueue_ordered_fill

Commit b44135daa372 introduced virtqueue_ordered_fill for
VIRTIO_F_IN_ORDER support but had a few issues:

* Conditional while loop used 'steps <= max_steps' but should've been
  'steps < max_steps' since reaching steps == max_steps would indicate
  that we didn't find an element, which is an error. Without this
  change, the code would attempt to read invalid data at an index
  outside of our search range.

* Incremented 'steps' using the next chain's ndescs instead of the
  current one.

This patch corrects the loop bounds and synchronizes 'steps' and index
increments.

We also add a defensive sanity check against malicious or invalid
descriptor counts to avoid a potential infinite loop and DoS.

Fixes: b44135daa372 ("virtio: virtqueue_ordered_fill - VIRTIO_F_IN_ORDER support")
Reported-by: terrynini <terrynini38514@gmail.com>
Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
Message-Id: <20250721150208.2409779-1-jonah.palmer@oracle.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 16 insertions, 6 deletions

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 2ab1d20..9a81ad9 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -938,18 +938,18 @@ static void virtqueue_packed_fill(VirtQueue *vq, const VirtQueueElement *elem,
 static void virtqueue_ordered_fill(VirtQueue *vq, const VirtQueueElement *elem,
                                    unsigned int len)
 {
-    unsigned int i, steps, max_steps;
+    unsigned int i, steps, max_steps, ndescs;
 
     i = vq->used_idx % vq->vring.num;
     steps = 0;
     /*
-     * We shouldn't need to increase 'i' by more than the distance
-     * between used_idx and last_avail_idx.
+     * We shouldn't need to increase 'i' by more than or equal to
+     * the distance between used_idx and last_avail_idx (max_steps).
      */
     max_steps = (vq->last_avail_idx - vq->used_idx) % vq->vring.num;
 
     /* Search for element in vq->used_elems */
-    while (steps <= max_steps) {
+    while (steps < max_steps) {
         /* Found element, set length and mark as filled */
         if (vq->used_elems[i].index == elem->index) {
             vq->used_elems[i].len = len;
@@ -957,8 +957,18 @@ static void virtqueue_ordered_fill(VirtQueue *vq, const VirtQueueElement *elem,
             break;
         }
 
-        i += vq->used_elems[i].ndescs;
-        steps += vq->used_elems[i].ndescs;
+        ndescs = vq->used_elems[i].ndescs;
+
+        /* Defensive sanity check */
+        if (unlikely(ndescs == 0 || ndescs > vq->vring.num)) {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "%s: %s invalid ndescs %u at position %u\n",
+                          __func__, vq->vdev->name, ndescs, i);
+            return;
+        }
+
+        i += ndescs;
+        steps += ndescs;
 
         if (i >= vq->vring.num) {
             i -= vq->vring.num;