[PATCH] virtio: fix use-after-free in unregister_virtio_device()

[Karl Mehltretter]

device_unregister() is device_del() plus put_device(). When the caller
holds no extra reference, that drops the last one and runs the release
callback, which for several transports frees the memory the embedded
struct virtio_device sits in. unregister_virtio_device() then calls
virtio_debug_device_exit(), which reads dev->debugfs_dir out of the freed
object.

Affected transports are the ones whose release callback frees and whose
remove path takes no reference: virtio_mmio, virtio_vdpa, virtio_uml,
mlxbf-tmfifo and virtio_ccw. virtio_pci is unaffected because
virtio_pci_remove() brackets the call with get_device() and put_device().

Remove the debugfs entries before the device can go away. They are only
accessed through the protected debugfs interface, so
debugfs_remove_recursive() waits for in-progress file operations before
returning. Tearing them down while the device is still alive is therefore
safe.

Reproduced on User-Mode Linux with CONFIG_KASAN and CONFIG_VIRTIO_DEBUG
by unbinding a virtio-uml device:

  BUG: KASAN: slab-use-after-free in virtio_debug_device_exit+0x36/0x4d
  Read of size 8 at addr 00000000616e0b10 by task init/1
   __asan_report_load8_noabort
   virtio_debug_device_exit+0x36/0x4d
   unregister_virtio_device+0x48/0x75
   virtio_uml_remove
   platform_remove
   device_release_driver_internal
   unbind_store

  Freed by task 1:
   kfree
   virtio_uml_release_dev
   device_release
   kobject_put
   put_device
   device_unregister
   unregister_virtio_device+0x48/0x75
   virtio_uml_remove

With this applied, the report is gone and unbind is clean.

Fixes: 96a8326d69ff ("virtio: add debugfs infrastructure to allow to debug virtio features")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
 drivers/virtio/virtio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 75bb4ffe3b877..b6c9e927bef57 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -604,8 +604,8 @@ void unregister_virtio_device(struct virtio_device *dev)
 {
 	int index = dev->index; /* save for after device release */
 
-	device_unregister(&dev->dev);
 	virtio_debug_device_exit(dev);
+	device_unregister(&dev->dev);
 	ida_free(&virtio_index_ida, index);
 }
 EXPORT_SYMBOL_GPL(unregister_virtio_device);

base-commit: 4477a78374a57c3809b172ad30cceabda48c47c6
-- 
2.53.0