CVE-2022-50555: tipc: fix a null-ptr-deref in tipc_topsrv_accept

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

tipc: fix a null-ptr-deref in tipctopsrvaccept

syzbot found a crash in tipctopsrvaccept:

KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f] Workqueue: tipcrcv tipctopsrvaccept RIP: 0010:kernelaccept+0x22d/0x350 net/socket.c:3487 Call Trace: <TASK> tipctopsrvaccept+0x197/0x280 net/tipc/topsrv.c:460 processonework+0x991/0x1610 kernel/workqueue.c:2289 workerthread+0x665/0x1080 kernel/workqueue.c:2436 kthread+0x2e4/0x3a0 kernel/kthread.c:376 retfromfork+0x1f/0x30 arch/x86/entry/entry64.S:306

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

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

Note that skcallbacklock is used to protect sk->skuserdata instead of srv->listener, and it should check srv in tipctopsrvlistenerdataready() instead. This also ensures that no more tipctopsrvaccept worker will be started after tipcconnclose() is called in tipctopsrvstop() where it sets sk->skuserdata to null.