net: qrtr: Fix a refcount bug in qrtr_recvmsg()

Syzbot reported a bug as following:

refcount_t: addition on 0; use-after-free.
...
RIP: 0010:refcount_warn_saturate+0x17c/0x1f0 lib/refcount.c:25
...
Call Trace:
 <TASK>
 __refcount_add include/linux/refcount.h:199 [inline]
 __refcount_inc include/linux/refcount.h:250 [inline]
 refcount_inc include/linux/refcount.h:267 [inline]
 kref_get include/linux/kref.h:45 [inline]
 qrtr_node_acquire net/qrtr/af_qrtr.c:202 [inline]
 qrtr_node_lookup net/qrtr/af_qrtr.c:398 [inline]
 qrtr_send_resume_tx net/qrtr/af_qrtr.c:1003 [inline]
 qrtr_recvmsg+0x85f/0x990 net/qrtr/af_qrtr.c:1070
 sock_recvmsg_nosec net/socket.c:1017 [inline]
 sock_recvmsg+0xe2/0x160 net/socket.c:1038
 qrtr_ns_worker+0x170/0x1700 net/qrtr/ns.c:688
 process_one_work+0x991/0x15c0 kernel/workqueue.c:2390
 worker_thread+0x669/0x1090 kernel/workqueue.c:2537

It occurs in the concurrent scenario of qrtr_recvmsg() and
qrtr_endpoint_unregister() as following:

	cpu0					cpu1
qrtr_recvmsg				qrtr_endpoint_unregister
qrtr_send_resume_tx			qrtr_node_release
qrtr_node_lookup			mutex_lock(&qrtr_node_lock)
spin_lock_irqsave(&qrtr_nodes_lock, )	refcount_dec_and_test(&node->ref) [node->ref == 0]
radix_tree_lookup [node != NULL]	__qrtr_node_release
qrtr_node_acquire			spin_lock_irqsave(&qrtr_nodes_lock, )
kref_get(&node->ref) [WARNING]		...
					mutex_unlock(&qrtr_node_lock)

Use qrtr_node_lock to protect qrtr_node_lookup() implementation, this
is actually improving the protection of node reference.

Fixes: 0a7e0d0e ("net: qrtr: Migrate node lookup tree to spinlock")
Reported-by: syzbot+a7492efaa5d61b51db23@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=a7492efaa5d61b51db23Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/qrtr/af_qrtr.c

@@ -393,10 +393,12 @@ static struct qrtr_node *qrtr_node_lookup(unsigned int nid)
 	struct qrtr_node *node;
 	unsigned long flags;
 
+	mutex_lock(&qrtr_node_lock);
 	spin_lock_irqsave(&qrtr_nodes_lock, flags);
 	node = radix_tree_lookup(&qrtr_nodes, nid);
 	node = qrtr_node_acquire(node);
 	spin_unlock_irqrestore(&qrtr_nodes_lock, flags);
+	mutex_unlock(&qrtr_node_lock);
 
 	return node;
 }