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Subject[PATCH][V5] tty: fix race between flush_to_ldisc and tty_open
Date
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 tty_open and
tty->driver_data may be NULL.

CPU0 CPU1
---- ----
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 get 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, and n_tty_receive_buf_common maybe calls uart_unthrottle,
so add the same check

Signed-off-by: Wang Li <wangli39@baidu.com>
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
---
V5: move check into uart_start from n_tty_receive_buf_common
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/serial/serial_core.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 5c01bb6d1c24..556f50aa1b58 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -130,6 +130,9 @@ static void uart_start(struct tty_struct *tty)
struct uart_port *port;
unsigned long flags;

+ if (!state)
+ return;
+
port = uart_port_lock(state, flags);
__uart_start(tty);
uart_port_unlock(port, flags);
@@ -727,6 +730,9 @@ static void uart_unthrottle(struct tty_struct *tty)
upstat_t mask = UPSTAT_SYNC_FIFO;
struct uart_port *port;

+ if (!state)
+ return;
+
port = uart_port_ref(state);
if (!port)
return;
--
2.16.2
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