greybus: Fix use-after-free bug in gb_interface_release due to race condition.

In the Linux kernel, the following vulnerability has been resolved:

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.