CVE-2022-50555

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

tipc: fix a null-ptr-deref in tipc_topsrv_accept

syzbot found a crash in tipc_topsrv_accept:

  KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
  Workqueue: tipc_rcv tipc_topsrv_accept
  RIP: 0010:kernel_accept+0x22d/0x350 net/socket.c:3487
  Call Trace:
   <TASK>
   tipc_topsrv_accept+0x197/0x280 net/tipc/topsrv.c:460
   process_one_work+0x991/0x1610 kernel/workqueue.c:2289
   worker_thread+0x665/0x1080 kernel/workqueue.c:2436
   kthread+0x2e4/0x3a0 kernel/kthread.c:376
   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306

It was caused by srv->listener that might be set to null by
tipc_topsrv_stop() in net .exit whereas it's still used in
tipc_topsrv_accept() worker.

srv->listener is protected by srv->idr_lock in tipc_topsrv_stop(), so add
a check for srv->listener under srv->idr_lock in tipc_topsrv_accept() to
avoid the null-ptr-deref. To ensure the lsock is not released during the
tipc_topsrv_accept(), move sock_release() after tipc_topsrv_work_stop()
where it's waiting until the tipc_topsrv_accept worker to be done.

Note that sk_callback_lock is used to protect sk->sk_user_data instead of
srv->listener, and it should check srv in tipc_topsrv_listener_data_ready()
instead. This also ensures that no more tipc_topsrv_accept worker will be
started after tipc_conn_close() is called in tipc_topsrv_stop() where it
sets sk->sk_user_data to null.