CVE-2021-47163: tipc: wait and exit until all work queues are done

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

tipc: wait and exit until all work queues are done

On some host, a crash could be triggered simply by repeating these commands several times:

# modprobe tipc # tipc bearer enable media udp name UDP1 localip 127.0.0.1 # rmmod tipc

[] BUG: unable to handle kernel paging request at ffffffffc096bb00 [] Workqueue: events 0xffffffffc096bb00 [] Call Trace: [] ? processonework+0x1a7/0x360 [] ? workerthread+0x30/0x390 [] ? createworker+0x1a0/0x1a0 [] ? kthread+0x116/0x130 [] ? kthreadflushworkfn+0x10/0x10 [] ? retfromfork+0x35/0x40

When removing the TIPC module, the UDP tunnel sock will be delayed to release in a work queue as sockrelease() can't be done in rtnllock(). If the work queue is schedule to run after the TIPC module is removed, kernel will crash as the work queue function cleanupbeareri() code no longer exists when trying to invoke it.

To fix it, this patch introduce a member wqcount in tipcnet to track the numbers of work queues in schedule, and wait and exit until all work queues are done in tipcexitnet().