greybus: Fix 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 relevant 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)

Fix it by canceling the work before kfree.

Signed-off-by: Sicong Huang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Sicong Huang

drivers/greybus/interface.c

@@ -693,6 +693,7 @@ static void gb_interface_release(struct device *dev)
 
 	trace_gb_interface_release(intf);
 
+	cancel_work_sync(&intf->mode_switch_work);
 	kfree(intf);
 }