ksmbd: fix UAF issue in ksmbd_tcp_new_connection()

The race is between the handling of a new TCP connection and
its disconnection. It leads to UAF on `struct tcp_transport` in
ksmbd_tcp_new_connection() function.

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-22991
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/connection.c

@@ -416,13 +416,7 @@ static void stop_sessions(void)
 again:
 	down_read(&conn_list_lock);
 	list_for_each_entry(conn, &conn_list, conns_list) {
-		struct task_struct *task;
-
 		t = conn->transport;
-		task = t->handler;
-		if (task)
-			ksmbd_debug(CONN, "Stop session handler %s/%d\n",
-				    task->comm, task_pid_nr(task));
 		ksmbd_conn_set_exiting(conn);
 		if (t->ops->shutdown) {
 			up_read(&conn_list_lock);

fs/smb/server/connection.h

@@ -135,7 +135,6 @@ struct ksmbd_transport_ops {
 struct ksmbd_transport {
 	struct ksmbd_conn		*conn;
 	struct ksmbd_transport_ops	*ops;
-	struct task_struct		*handler;
 };
 
 #define KSMBD_TCP_RECV_TIMEOUT	(7 * HZ)

fs/smb/server/transport_rdma.c

@@ -2039,6 +2039,7 @@ static bool rdma_frwr_is_supported(struct ib_device_attr *attrs)
 static int smb_direct_handle_connect_request(struct rdma_cm_id *new_cm_id)
 {
 	struct smb_direct_transport *t;
+	struct task_struct *handler;
 	int ret;
 
 	if (!rdma_frwr_is_supported(&new_cm_id->device->attrs)) {
@@ -2056,11 +2057,11 @@ static int smb_direct_handle_connect_request(struct rdma_cm_id *new_cm_id)
 	if (ret)
 		goto out_err;
 
-	KSMBD_TRANS(t)->handler = kthread_run(ksmbd_conn_handler_loop,
-					      KSMBD_TRANS(t)->conn, "ksmbd:r%u",
-					      smb_direct_port);
-	if (IS_ERR(KSMBD_TRANS(t)->handler)) {
-		ret = PTR_ERR(KSMBD_TRANS(t)->handler);
+	handler = kthread_run(ksmbd_conn_handler_loop,
+			      KSMBD_TRANS(t)->conn, "ksmbd:r%u",
+			      smb_direct_port);
+	if (IS_ERR(handler)) {
+		ret = PTR_ERR(handler);
 		pr_err("Can't start thread\n");
 		goto out_err;
 	}

fs/smb/server/transport_tcp.c

@@ -185,6 +185,7 @@ static int ksmbd_tcp_new_connection(struct socket *client_sk)
 	struct sockaddr *csin;
 	int rc = 0;
 	struct tcp_transport *t;
+	struct task_struct *handler;
 
 	t = alloc_transport(client_sk);
 	if (!t) {
@@ -199,13 +200,13 @@ static int ksmbd_tcp_new_connection(struct socket *client_sk)
 		goto out_error;
 	}
 
-	KSMBD_TRANS(t)->handler = kthread_run(ksmbd_conn_handler_loop,
-					      KSMBD_TRANS(t)->conn,
-					      "ksmbd:%u",
-					      ksmbd_tcp_get_port(csin));
-	if (IS_ERR(KSMBD_TRANS(t)->handler)) {
+	handler = kthread_run(ksmbd_conn_handler_loop,
+			      KSMBD_TRANS(t)->conn,
+			      "ksmbd:%u",
+			      ksmbd_tcp_get_port(csin));
+	if (IS_ERR(handler)) {
 		pr_err("cannot start conn thread\n");
-		rc = PTR_ERR(KSMBD_TRANS(t)->handler);
+		rc = PTR_ERR(handler);
 		free_transport(t);
 	}
 	return rc;