[PATCH][v4] tty: fix race between flush_to_ldisc and tty_open

[Li RongQing]

There still is a race window after the commit b027e2298bd588
("tty: fix data race between tty_init_dev and flush of buf"),
and we encountered this crash issue if receive_buf call comes
before tty initialization completes in n_tty_open and
tty->driver_data may be NULL.

CPU0                                    CPU1
----                                    ----
                                 n_tty_open
                                   tty_init_dev
                                     tty_ldisc_unlock
                                       schedule
flush_to_ldisc
 receive_buf
  tty_port_default_receive_buf
   tty_ldisc_receive_buf
    n_tty_receive_buf_common
      __receive_buf
       uart_flush_chars
        uart_start
        /*tty->driver_data is NULL*/
                                   tty->ops->open
                                   /*init tty->driver_data*/

it can be fixed by extending ldisc semaphore lock in tty_init_dev
to driver_data initialized completely after tty->ops->open(), but
this will lead to put lock on one function and unlock in some other
function, and hard to maintain, so fix this race only by checking
tty->driver_data when receiving, and return if tty->driver_data
is NULL

Signed-off-by: Wang Li <wangli39@xxxxxxxxx>
Signed-off-by: Zhang Yu <zhangyu31@xxxxxxxxx>
Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
---
V4: add version information
V3: not used ldisc semaphore lock, only checking tty->driver_data with NULL
V2: fix building error by EXPORT_SYMBOL tty_ldisc_unlock
V1: extend ldisc lock to protect that tty->driver_data is inited 

drivers/tty/tty_port.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 044c3cbdcfa4..86d0bec38322 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -31,6 +31,9 @@ static int tty_port_default_receive_buf(struct tty_port *port,
 	if (!tty)
 		return 0;
 
+	if (!tty->driver_data)
+		return 0;
+
 	disc = tty_ldisc_ref(tty);
 	if (!disc)
 		return 0;
-- 
2.16.2