[PATCH][v3] tty: fix race between flush_to_ldisc and tty_open

[PATCH][v3] 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@baidu.com>
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
---
 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

Re: [PATCH][v3] tty: fix race between flush_to_ldisc and tty_open

[Greg KH]

On Fri, Jan 18, 2019 at 04:56:04PM +0800, Li RongQing wrote:
> 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@baidu.com>
> Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>  drivers/tty/tty_port.c | 3 +++
>  1 file changed, 3 insertions(+)

What changed in this version, from the other versions?  That is always
to be listed below the --- line, as the documentation states.

Please fix up and do a v4 with all of this information.

thanks,

greg k-h