Re: [PATCH][V5] tty: fix race between flush_to_ldisc and tty_open

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jan 31, 2019 at 05:43:16PM +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 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


Hm, I wrote this patch, not you, right?  So shouldn't I get the
credit/blame for it?  :)

Also, this is a bug in the serial code, not necessarily the tty layer,
so the subject should change...

And you did test this, right?

thanks,

greg k-h