Bug report: greybus/interface.c: use-after-free bug in gb_interface_release due to race condition

[sicong <congei42@163.com>]

greybus/interface.c: use-after-free bug in gb_interface_release due to
race condition.

In gb_interface_create, &intf->mode_switch_completion is bound with 
gb_interface_mode_switch_work. Then it will be started by 
gb_interface_request_mode_switch. Here is the code.
if (!queue_work(system_long_wq, &intf->mode_switch_work)) {
	...
}

If we call gb_interface_release to make cleanup, there may be an 
unfinished work. This function will call kfree to free the object 
"intf". However, if gb_interface_mode_switch_work is scheduled to 
run after kfree, it may cause use-after-free error as 
gb_interface_mode_switch_work will use the object "intf". 
The possible execution flow that may lead to the issue is as follows:

CPU0                            CPU1

                            |   gb_interface_create
                            |   gb_interface_request_mode_switch
gb_interface_release        |
kfree(intf) (free)          |
                            |   gb_interface_mode_switch_work
                            |   mutex_lock(&intf->mutex) (use)   

This bug may be fixed by adding the following code before kfree.
cancel_work_sync(&intf->mode_switch_work);

Best regards,
Sicong Huang