Re: [PATCH v2 4/5] vhost: check vring address before calling unmap

[Jason Wang]

On 2020/5/11 下午5:11, Dima Stepanov wrote:
On Mon, May 11, 2020 at 11:05:58AM +0800, Jason Wang wrote:
On 2020/4/30 下午9:36, Dima Stepanov wrote:
Since disconnect can happen at any time during initialization not all
vring buffers (for instance used vring) can be intialized successfully.
If the buffer was not initialized then vhost_memory_unmap call will lead
to SIGSEGV. Add checks for the vring address value before calling unmap.
Also add assert() in the vhost_memory_unmap() routine.

Signed-off-by: Dima Stepanov <dimas...@yandex-team.ru>
---
  hw/virtio/vhost.c | 27 +++++++++++++++++++++------
  1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index ddbdc53..3ee50c4 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -314,6 +314,8 @@ static void vhost_memory_unmap(struct vhost_dev *dev, void 
*buffer,
                                 hwaddr len, int is_write,
                                 hwaddr access_len)
  {
+    assert(buffer);
+
      if (!vhost_dev_has_iommu(dev)) {
          cpu_physical_memory_unmap(buffer, len, is_write, access_len);
      }
@@ -1132,12 +1134,25 @@ static void vhost_virtqueue_stop(struct vhost_dev *dev,
                                                  vhost_vq_index);
      }
-    vhost_memory_unmap(dev, vq->used, virtio_queue_get_used_size(vdev, idx),
-                       1, virtio_queue_get_used_size(vdev, idx));
-    vhost_memory_unmap(dev, vq->avail, virtio_queue_get_avail_size(vdev, idx),
-                       0, virtio_queue_get_avail_size(vdev, idx));
-    vhost_memory_unmap(dev, vq->desc, virtio_queue_get_desc_size(vdev, idx),
-                       0, virtio_queue_get_desc_size(vdev, idx));
+    /*
+     * Since the vhost-user disconnect can happen during initialization
+     * check if vring was initialized, before making unmap.
+     */
+    if (vq->used) {
+        vhost_memory_unmap(dev, vq->used,
+                           virtio_queue_get_used_size(vdev, idx),
+                           1, virtio_queue_get_used_size(vdev, idx));
+    }
+    if (vq->avail) {
+        vhost_memory_unmap(dev, vq->avail,
+                           virtio_queue_get_avail_size(vdev, idx),
+                           0, virtio_queue_get_avail_size(vdev, idx));
+    }
+    if (vq->desc) {
+        vhost_memory_unmap(dev, vq->desc,
+                           virtio_queue_get_desc_size(vdev, idx),
+                           0, virtio_queue_get_desc_size(vdev, idx));
+    }

Any reason not checking hdev->started instead? vhost_dev_start() will set it
to true if virtqueues were correctly mapped.

Thanks
Well i see it a little bit different:
  - vhost_dev_start() sets hdev->started to true before starting
    virtqueues
  - vhost_virtqueue_start() maps all the memory
If we hit the vhost disconnect at the start of the
vhost_virtqueue_start(), for instance for this call:
   r = dev->vhost_ops->vhost_set_vring_base(dev, &state);
Then we will call vhost_user_blk_disconnect:
   vhost_user_blk_disconnect()->
     vhost_user_blk_stop()->
       vhost_dev_stop()->
         vhost_virtqueue_stop()
As a result we will come in this routine with the hdev->started still
set to true, but if used/avail/desc fields still uninitialized and set
to 0.


I may miss something, but consider both vhost_dev_start() and 
vhost_user_blk_disconnect() were serialized in main loop. Can this 
really happen?

Thanks




  }
  static void vhost_eventfd_add(MemoryListener *listener,