Re: [Qemu-devel] [PATCH] qdev: Fix use after free in qdev_init_nofail error path

[Igor Mammedov]

On Tue,  2 Aug 2016 11:41:41 +0800
Fam Zheng <address@hidden> wrote:

> Since 69382d8b (qdev: Fix object reference leak in case device.realize()
> fails), object_property_set_bool could release the object. The error
> path wants the type name, so hold an reference before realizing it.
> 
> Cc: Igor Mammedov <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  hw/core/qdev.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index ee4a083..5783442 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -354,12 +354,14 @@ void qdev_init_nofail(DeviceState *dev)
>  
>      assert(!dev->realized);
>  
> +    object_ref(OBJECT(dev));
>      object_property_set_bool(OBJECT(dev), true, "realized", &err);
>      if (err) {
>          error_reportf_err(err, "Initialization of device %s failed: ",
>                            object_get_typename(OBJECT(dev)));
>          exit(1);
>      }
> +    object_unref(OBJECT(dev));
>  }
>  
>  void qdev_machine_creation_done(void)

I'm not sure that this is the right fix, commit 69382d8b only affects
reference created by realize() itself.
Probably reference counting wrong somewhere else,
for typical device call sequence is following:

 qdev_create() {
    object_new() -> ref == 1
    qdev_set_parent_bus() -> ref == 2
    object_unref() -> ref == 1
 } -> ref == 1
 
 do property settings and other stuff ...

 
 qdev_init_nofail() { called with ref == 1
    object_property_set_bool(true, "realized")
    if error:
          ref == 1
    else:
          ref == 2 (+1 for implicitly assigned parent)
 }