tipc: set con sock in tipc_conn_alloc

A crash was reported by Wei Chen:

  BUG: kernel NULL pointer dereference, address: 0000000000000018
  RIP: 0010:tipc_conn_close+0x12/0x100
  Call Trace:
   tipc_topsrv_exit_net+0x139/0x320
   ops_exit_list.isra.9+0x49/0x80
   cleanup_net+0x31a/0x540
   process_one_work+0x3fa/0x9f0
   worker_thread+0x42/0x5c0

It was caused by !con->sock in tipc_conn_close(). In tipc_topsrv_accept(),
con is allocated in conn_idr then its sock is set:

  con = tipc_conn_alloc();
  ...                    <----[1]
  con->sock = newsock;

If tipc_conn_close() is called in anytime of [1], the null-pointer-def
is triggered by con->sock->sk due to con->sock is not yet set.

This patch fixes it by moving the con->sock setting to tipc_conn_alloc()
under s->idr_lock. So that con->sock can never be NULL when getting the
con from s->conn_idr. It will be also safer to move con->server and flag
CF_CONNECTED setting under s->idr_lock, as they should all be set before
tipc_conn_alloc() is called.

Fixes: c5fa7b3c ("tipc: introduce new TIPC server infrastructure")
Reported-by: Wei Chen <harperchen1110@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/tipc/topsrv.c

@@ -176,7 +176,7 @@ static void tipc_conn_close(struct tipc_conn *con)
 	conn_put(con);
 }
 
-static struct tipc_conn *tipc_conn_alloc(struct tipc_topsrv *s)
+static struct tipc_conn *tipc_conn_alloc(struct tipc_topsrv *s, struct socket *sock)
 {
 	struct tipc_conn *con;
 	int ret;
@@ -202,10 +202,11 @@ static struct tipc_conn *tipc_conn_alloc(struct tipc_topsrv *s)
 	}
 	con->conid = ret;
 	s->idr_in_use++;
-	spin_unlock_bh(&s->idr_lock);
 
 	set_bit(CF_CONNECTED, &con->flags);
 	con->server = s;
+	con->sock = sock;
+	spin_unlock_bh(&s->idr_lock);
 
 	return con;
 }
@@ -467,7 +468,7 @@ static void tipc_topsrv_accept(struct work_struct *work)
 		ret = kernel_accept(lsock, &newsock, O_NONBLOCK);
 		if (ret < 0)
 			return;
-		con = tipc_conn_alloc(srv);
+		con = tipc_conn_alloc(srv, newsock);
 		if (IS_ERR(con)) {
 			ret = PTR_ERR(con);
 			sock_release(newsock);
@@ -479,7 +480,6 @@ static void tipc_topsrv_accept(struct work_struct *work)
 		newsk->sk_data_ready = tipc_conn_data_ready;
 		newsk->sk_write_space = tipc_conn_write_space;
 		newsk->sk_user_data = con;
-		con->sock = newsock;
 		write_unlock_bh(&newsk->sk_callback_lock);
 
 		/* Wake up receive process in case of 'SYN+' message */
@@ -577,12 +577,11 @@ bool tipc_topsrv_kern_subscr(struct net *net, u32 port, u32 type, u32 lower,
 	sub.filter = filter;
 	*(u64 *)&sub.usr_handle = (u64)port;
 
-	con = tipc_conn_alloc(tipc_topsrv(net));
+	con = tipc_conn_alloc(tipc_topsrv(net), NULL);
 	if (IS_ERR(con))
 		return false;
 
 	*conid = con->conid;
-	con->sock = NULL;
 	rc = tipc_conn_rcv_sub(tipc_topsrv(net), con, &sub);
 	if (rc >= 0)
 		return true;