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

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/08/2016 09:55, Igor Mammedov wrote:
> On Tue,  2 Aug 2016 11:41:41 +0800
> Fam Zheng <famz@redhat.com> 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 <imammedo@redhat.com>
>> Signed-off-by: Fam Zheng <famz@redhat.com>
>> ---
>>  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

If there is an error and the device was unattached, you get here:

    if (unattached_parent) {
        object_unparent(OBJECT(dev));
        unattached_count--;
    }

and object_unparent undoes qdev_set_parent_bus so that the refcount
drops to 0.

Paolo

>     else:
>           ref == 2 (+1 for implicitly assigned parent)
>  }
> 
>