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Subject: Re: [PATCH][v4] tty: fix race between flush_to_ldisc and tty_open
To:     Li RongQing <lirongqing@baidu.com>, gregkh@linuxfoundation.org,
        jslaby@suse.com, linux-kernel@vger.kernel.org
References: <1547803637-29135-1-git-send-email-lirongqing@baidu.com>
From:   "Kohli, Gaurav" <gkohli@codeaurora.org>
Message-ID: <ae44b1c5-4d33-110b-a4da-f526d156be2a@codeaurora.org>
Date:   Fri, 18 Jan 2019 18:20:33 +0530
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On 1/18/2019 2:57 PM, 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;
> +
>   	disc = tty_ldisc_ref(tty);
>   	if (!disc)
>   		return 0;
>
Acked-by: Gaurav Kohli <gkohli@codeaurora.org>

It looks good to me w.r.t previous approach, but Let's Maintainer decide 
once.

Regards
Gaurav
-- 
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