net: qrtr: ns: Protect radix_tree_deref_slot() using rcu read locks

The rcu read locks are needed to avoid potential race condition while
dereferencing radix tree from multiple threads. The issue was identified
by syzbot. Below is the crash report:

=============================
WARNING: suspicious RCU usage
5.7.0-syzkaller #0 Not tainted
-----------------------------
include/linux/radix-tree.h:176 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
2 locks held by kworker/u4:1/21:
 #0: ffff88821b097938 ((wq_completion)qrtr_ns_handler){+.+.}-{0:0}, at: spin_unlock_irq include/linux/spinlock.h:403 [inline]
 #0: ffff88821b097938 ((wq_completion)qrtr_ns_handler){+.+.}-{0:0}, at: process_one_work+0x6df/0xfd0 kernel/workqueue.c:2241
 #1: ffffc90000dd7d80 ((work_completion)(&qrtr_ns.work)){+.+.}-{0:0}, at: process_one_work+0x71e/0xfd0 kernel/workqueue.c:2243

stack backtrace:
CPU: 0 PID: 21 Comm: kworker/u4:1 Not tainted 5.7.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: qrtr_ns_handler qrtr_ns_worker
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1e9/0x30e lib/dump_stack.c:118
 radix_tree_deref_slot include/linux/radix-tree.h:176 [inline]
 ctrl_cmd_new_lookup net/qrtr/ns.c:558 [inline]
 qrtr_ns_worker+0x2aff/0x4500 net/qrtr/ns.c:674
 process_one_work+0x76e/0xfd0 kernel/workqueue.c:2268
 worker_thread+0xa7f/0x1450 kernel/workqueue.c:2414
 kthread+0x353/0x380 kernel/kthread.c:268

Fixes: 0c2204a4 ("net: qrtr: Migrate nameservice to kernel from userspace")
Reported-and-tested-by: syzbot+0f84f6eed90503da72fc@syzkaller.appspotmail.com
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/qrtr/ns.c

@@ -193,12 +193,13 @@ static int announce_servers(struct sockaddr_qrtr *sq)
 	struct qrtr_server *srv;
 	struct qrtr_node *node;
 	void __rcu **slot;
-	int ret;
+	int ret = 0;
 
 	node = node_get(qrtr_ns.local_node);
 	if (!node)
 		return 0;
 
+	rcu_read_lock();
 	/* Announce the list of servers registered in this node */
 	radix_tree_for_each_slot(slot, &node->servers, &iter, 0) {
 		srv = radix_tree_deref_slot(slot);
@@ -206,11 +207,14 @@ static int announce_servers(struct sockaddr_qrtr *sq)
 		ret = service_announce_new(sq, srv);
 		if (ret < 0) {
 			pr_err("failed to announce new service\n");
-			return ret;
+			goto err_out;
 		}
 	}
 
-	return 0;
+err_out:
+	rcu_read_unlock();
+
+	return ret;
 }
 
 static struct qrtr_server *server_add(unsigned int service,
@@ -335,7 +339,7 @@ static int ctrl_cmd_bye(struct sockaddr_qrtr *from)
 	struct qrtr_node *node;
 	void __rcu **slot;
 	struct kvec iv;
-	int ret;
+	int ret = 0;
 
 	iv.iov_base = &pkt;
 	iv.iov_len = sizeof(pkt);
@@ -344,11 +348,13 @@ static int ctrl_cmd_bye(struct sockaddr_qrtr *from)
 	if (!node)
 		return 0;
 
+	rcu_read_lock();
 	/* Advertise removal of this client to all servers of remote node */
 	radix_tree_for_each_slot(slot, &node->servers, &iter, 0) {
 		srv = radix_tree_deref_slot(slot);
 		server_del(node, srv->port);
 	}
+	rcu_read_unlock();
 
 	/* Advertise the removal of this client to all local servers */
 	local_node = node_get(qrtr_ns.local_node);
@@ -359,6 +365,7 @@ static int ctrl_cmd_bye(struct sockaddr_qrtr *from)
 	pkt.cmd = cpu_to_le32(QRTR_TYPE_BYE);
 	pkt.client.node = cpu_to_le32(from->sq_node);
 
+	rcu_read_lock();
 	radix_tree_for_each_slot(slot, &local_node->servers, &iter, 0) {
 		srv = radix_tree_deref_slot(slot);
 
@@ -372,11 +379,14 @@ static int ctrl_cmd_bye(struct sockaddr_qrtr *from)
 		ret = kernel_sendmsg(qrtr_ns.sock, &msg, &iv, 1, sizeof(pkt));
 		if (ret < 0) {
 			pr_err("failed to send bye cmd\n");
-			return ret;
+			goto err_out;
 		}
 	}
 
-	return 0;
+err_out:
+	rcu_read_unlock();
+
+	return ret;
 }
 
 static int ctrl_cmd_del_client(struct sockaddr_qrtr *from,
@@ -394,7 +404,7 @@ static int ctrl_cmd_del_client(struct sockaddr_qrtr *from,
 	struct list_head *li;
 	void __rcu **slot;
 	struct kvec iv;
-	int ret;
+	int ret = 0;
 
 	iv.iov_base = &pkt;
 	iv.iov_len = sizeof(pkt);
@@ -434,6 +444,7 @@ static int ctrl_cmd_del_client(struct sockaddr_qrtr *from,
 	pkt.client.node = cpu_to_le32(node_id);
 	pkt.client.port = cpu_to_le32(port);
 
+	rcu_read_lock();
 	radix_tree_for_each_slot(slot, &local_node->servers, &iter, 0) {
 		srv = radix_tree_deref_slot(slot);
 
@@ -447,11 +458,14 @@ static int ctrl_cmd_del_client(struct sockaddr_qrtr *from,
 		ret = kernel_sendmsg(qrtr_ns.sock, &msg, &iv, 1, sizeof(pkt));
 		if (ret < 0) {
 			pr_err("failed to send del client cmd\n");
-			return ret;
+			goto err_out;
 		}
 	}
 
-	return 0;
+err_out:
+	rcu_read_unlock();
+
+	return ret;
 }
 
 static int ctrl_cmd_new_server(struct sockaddr_qrtr *from,
@@ -554,6 +568,7 @@ static int ctrl_cmd_new_lookup(struct sockaddr_qrtr *from,
 	filter.service = service;
 	filter.instance = instance;
 
+	rcu_read_lock();
 	radix_tree_for_each_slot(node_slot, &nodes, &node_iter, 0) {
 		node = radix_tree_deref_slot(node_slot);
 
@@ -568,6 +583,7 @@ static int ctrl_cmd_new_lookup(struct sockaddr_qrtr *from,
 			lookup_notify(from, srv, true);
 		}
 	}
+	rcu_read_unlock();
 
 	/* Empty notification, to indicate end of listing */
 	lookup_notify(from, NULL, true);