[net-next 1/1] tipc: fix race condition at topology server receive

[Jon Maloy <jon.maloy@ericsson.com>]

We have identified a race condition during reception of socket
events and messages in the topology server.

- The function tipc_close_conn() is releasing the corresponding
  struct tipc_subscriber instance without considering that there
  may still be items in the receive work queue. When those are
  scheduled, in the function tipc_receive_from_work(), they are
  using the subscriber pointer stored in struct tipc_conn, without
  first checking if this is valid or not. This will sometimes
  lead to crashes, as the next call of tipc_conn_recvmsg() will
  access the now deleted item.
  We fix this by making the usage of this pointer conditional on
  whether the connection is active or not. I.e., we check the condition
  test_bit(CF_CONNECTED) before making the call tipc_conn_recvmsg().

- Since the two functions may be running on different cores, the
  condition test described above is not enough. tipc_close_conn()
  may come in between and delete the subscriber item after the condition
  test is done, but before tipc_conn_recv_msg() is finished. This
  happens less frequently than the problem described above, but leads
  to the same symptoms.

  We fix this by using the existing sk_callback_lock for mutual
  exclusion in the two functions. In addition, we have to move
  a call to tipc_conn_terminate() outside the mentioned lock to
  avoid deadlock.

Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
---
 net/tipc/server.c | 70 +++++++++++++++++++++++++++++--------------------------
 net/tipc/server.h |  6 ++---
 net/tipc/subscr.c | 21 +++++++++--------
 3 files changed, 51 insertions(+), 46 deletions(-)

diff --git a/net/tipc/server.c b/net/tipc/server.c
index 8ee5e86..c0d331f 100644
--- a/net/tipc/server.c
+++ b/net/tipc/server.c
@@ -132,10 +132,11 @@ static struct tipc_conn *tipc_conn_lookup(struct tipc_server *s, int conid)
 
 	spin_lock_bh(&s->idr_lock);
 	con = idr_find(&s->conn_idr, conid);
-	if (con && test_bit(CF_CONNECTED, &con->flags))
-		conn_get(con);
-	else
-		con = NULL;
+	if (con) {
+		if (!test_bit(CF_CONNECTED, &con->flags) ||
+		    !kref_get_unless_zero(&con->kref))
+			con = NULL;
+	}
 	spin_unlock_bh(&s->idr_lock);
 	return con;
 }
@@ -183,35 +184,28 @@ static void tipc_register_callbacks(struct socket *sock, struct tipc_conn *con)
 	write_unlock_bh(&sk->sk_callback_lock);
 }
 
-static void tipc_unregister_callbacks(struct tipc_conn *con)
-{
-	struct sock *sk = con->sock->sk;
-
-	write_lock_bh(&sk->sk_callback_lock);
-	sk->sk_user_data = NULL;
-	write_unlock_bh(&sk->sk_callback_lock);
-}
-
 static void tipc_close_conn(struct tipc_conn *con)
 {
 	struct tipc_server *s = con->server;
+	struct sock *sk = con->sock->sk;
+	bool disconnect = false;
 
-	if (test_and_clear_bit(CF_CONNECTED, &con->flags)) {
-		if (con->sock)
-			tipc_unregister_callbacks(con);
-
+	write_lock_bh(&sk->sk_callback_lock);
+	disconnect = test_and_clear_bit(CF_CONNECTED, &con->flags);
+	if (disconnect) {
+		sk->sk_user_data = NULL;
 		if (con->conid)
 			s->tipc_conn_release(con->conid, con->usr_data);
-
-		/* We shouldn't flush pending works as we may be in the
-		 * thread. In fact the races with pending rx/tx work structs
-		 * are harmless for us here as we have already deleted this
-		 * connection from server connection list.
-		 */
-		if (con->sock)
-			kernel_sock_shutdown(con->sock, SHUT_RDWR);
-		conn_put(con);
 	}
+	write_unlock_bh(&sk->sk_callback_lock);
+
+	/* Handle concurrent calls from sending and receiving threads */
+	if (!disconnect)
+		return;
+
+	/* Don't flush pending works, -just let them expire */
+	kernel_sock_shutdown(con->sock, SHUT_RDWR);
+	conn_put(con);
 }
 
 static struct tipc_conn *tipc_alloc_conn(struct tipc_server *s)
@@ -248,9 +242,10 @@ static struct tipc_conn *tipc_alloc_conn(struct tipc_server *s)
 
 static int tipc_receive_from_sock(struct tipc_conn *con)
 {
-	struct msghdr msg = {};
 	struct tipc_server *s = con->server;
+	struct sock *sk = con->sock->sk;
 	struct sockaddr_tipc addr;
+	struct msghdr msg = {};
 	struct kvec iov;
 	void *buf;
 	int ret;
@@ -271,12 +266,15 @@ static int tipc_receive_from_sock(struct tipc_conn *con)
 		goto out_close;
 	}
 
-	s->tipc_conn_recvmsg(sock_net(con->sock->sk), con->conid, &addr,
-			     con->usr_data, buf, ret);
-
+	read_lock_bh(&sk->sk_callback_lock);
+	if (test_bit(CF_CONNECTED, &con->flags))
+		ret = s->tipc_conn_recvmsg(sock_net(con->sock->sk), con->conid,
+					   &addr, con->usr_data, buf, ret);
+	read_unlock_bh(&sk->sk_callback_lock);
 	kmem_cache_free(s->rcvbuf_cache, buf);
-
-	return 0;
+	if (ret < 0)
+		tipc_conn_terminate(s, con->conid);
+	return ret;
 
 out_close:
 	if (ret != -EWOULDBLOCK)
@@ -525,11 +523,17 @@ bool tipc_topsrv_kern_subscr(struct net *net, u32 port, u32 type, u32 lower,
 void tipc_topsrv_kern_unsubscr(struct net *net, int conid)
 {
 	struct tipc_conn *con;
+	struct tipc_server *srv;
 
 	con = tipc_conn_lookup(tipc_topsrv(net), conid);
 	if (!con)
 		return;
-	tipc_close_conn(con);
+
+	test_and_clear_bit(CF_CONNECTED, &con->flags);
+	srv = con->server;
+	if (con->conid)
+		srv->tipc_conn_release(con->conid, con->usr_data);
+	conn_put(con);
 	conn_put(con);
 }
 
diff --git a/net/tipc/server.h b/net/tipc/server.h
index 17f49ee..64df751 100644
--- a/net/tipc/server.h
+++ b/net/tipc/server.h
@@ -74,9 +74,9 @@ struct tipc_server {
 	int max_rcvbuf_size;
 	void *(*tipc_conn_new)(int conid);
 	void (*tipc_conn_release)(int conid, void *usr_data);
-	void (*tipc_conn_recvmsg)(struct net *net, int conid,
-				  struct sockaddr_tipc *addr, void *usr_data,
-				  void *buf, size_t len);
+	int (*tipc_conn_recvmsg)(struct net *net, int conid,
+				 struct sockaddr_tipc *addr, void *usr_data,
+				 void *buf, size_t len);
 	struct sockaddr_tipc *saddr;
 	char name[TIPC_SERVER_NAME_LEN];
 	int imp;
diff --git a/net/tipc/subscr.c b/net/tipc/subscr.c
index 44df528..68e2647 100644
--- a/net/tipc/subscr.c
+++ b/net/tipc/subscr.c
@@ -289,17 +289,16 @@ static struct tipc_subscription *tipc_subscrp_create(struct net *net,
 	return sub;
 }
 
-static void tipc_subscrp_subscribe(struct net *net, struct tipc_subscr *s,
-				   struct tipc_subscriber *subscriber, int swap,
-				   bool status)
+static int tipc_subscrp_subscribe(struct net *net, struct tipc_subscr *s,
+				  struct tipc_subscriber *subscriber, int swap,
+				  bool status)
 {
-	struct tipc_net *tn = net_generic(net, tipc_net_id);
 	struct tipc_subscription *sub = NULL;
 	u32 timeout;
 
 	sub = tipc_subscrp_create(net, s, swap);
 	if (!sub)
-		return tipc_conn_terminate(tn->topsrv, subscriber->conid);
+		return -1;
 
 	spin_lock_bh(&subscriber->lock);
 	list_add(&sub->subscrp_list, &subscriber->subscrp_list);
@@ -313,6 +312,7 @@ static void tipc_subscrp_subscribe(struct net *net, struct tipc_subscr *s,
 
 	if (timeout != TIPC_WAIT_FOREVER)
 		mod_timer(&sub->timer, jiffies + msecs_to_jiffies(timeout));
+	return 0;
 }
 
 /* Handle one termination request for the subscriber */
@@ -322,9 +322,9 @@ static void tipc_subscrb_release_cb(int conid, void *usr_data)
 }
 
 /* Handle one request to create a new subscription for the subscriber */
-static void tipc_subscrb_rcv_cb(struct net *net, int conid,
-				struct sockaddr_tipc *addr, void *usr_data,
-				void *buf, size_t len)
+static int tipc_subscrb_rcv_cb(struct net *net, int conid,
+			       struct sockaddr_tipc *addr, void *usr_data,
+			       void *buf, size_t len)
 {
 	struct tipc_subscriber *subscriber = usr_data;
 	struct tipc_subscr *s = (struct tipc_subscr *)buf;
@@ -338,10 +338,11 @@ static void tipc_subscrb_rcv_cb(struct net *net, int conid,
 	/* Detect & process a subscription cancellation request */
 	if (s->filter & htohl(TIPC_SUB_CANCEL, swap)) {
 		s->filter &= ~htohl(TIPC_SUB_CANCEL, swap);
-		return tipc_subscrp_cancel(s, subscriber);
+		tipc_subscrp_cancel(s, subscriber);
+		return 0;
 	}
 	status = !(s->filter & htohl(TIPC_SUB_NO_STATUS, swap));
-	tipc_subscrp_subscribe(net, s, subscriber, swap, status);
+	return tipc_subscrp_subscribe(net, s, subscriber, swap, status);
 }
 
 /* Handle one request to establish a new subscriber */
-- 
2.1.4