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

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Jan 18, 2019 at 05:27:17PM +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>
> ---
> 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;
> +

How is this working?  What is setting driver_data to NULL to "stop" this
race?

There's no requirement that a tty driver set this field to NULL when it
is "done" with the tty device, so I think you are just getting lucky in
that your specific driver happens to be doing this.

What driver are you testing this against?

thanks,

greg k-h