hw/virtio-iommu: Fix potential OOB access in virtio_iommu_handle_command()

In the virtio_iommu_handle_command() when a PROBE request is handled,
output_size takes a value greater than the tail size and on a subsequent
iteration we can get a stack out-of-band access. Initialize the
output_size on each iteration.

The issue was found with ASAN. Credits to:
Yiming Tao(Zhejiang University)
Gaoning Pan(Zhejiang University)

Fixes: 1733eebb9e7 ("virtio-iommu: Implement RESV_MEM probe request")
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Mauro Matteo Cascella <mcascell@redhat.com>
Cc: qemu-stable@nongnu.org

Message-Id: <20230717162126.11693-1-eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/hw/virtio/virtio-iommu.c b/hw/virtio/virtio-iommu.c
index 201127c..4dcf1d5 100644
--- a/hw/virtio/virtio-iommu.c
+++ b/hw/virtio/virtio-iommu.c
@@ -728,13 +728,15 @@ static void virtio_iommu_handle_command(VirtIODevice *vdev, VirtQueue *vq)
     VirtIOIOMMU *s = VIRTIO_IOMMU(vdev);
     struct virtio_iommu_req_head head;
     struct virtio_iommu_req_tail tail = {};
-    size_t output_size = sizeof(tail), sz;
     VirtQueueElement *elem;
     unsigned int iov_cnt;
     struct iovec *iov;
     void *buf = NULL;
+    size_t sz;
 
     for (;;) {
+        size_t output_size = sizeof(tail);
+
         elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
         if (!elem) {
             return;