Re: tty: fix a possible hang on tty device

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On Wed, Jun 01, 2022 at 09:39:27PM +0800, cael wrote:
> From: cael <juanfengpy@xxxxxxxxx>
> Subject: [PATCH v2] tty: fix a possible hang on tty device
> 
> We have met a hang on pty device, the reader was blocking
> at epoll on master side, the writer was sleeping at wait_woken
> inside n_tty_write on slave side, and the write buffer on
> tty_port was full, we found that the reader and writer would
> never be woken again and block forever.
> 
> The problem was caused by a race between reader and kworker:
> n_tty_read(reader):  n_tty_receive_buf_common(kworker):
>                     |room = N_TTY_BUF_SIZE - (ldata->read_head - tail)
>                     |room <= 0
> copy_from_read_buf()|
> n_tty_kick_worker() |
>                     |ldata->no_room = true
> 
> After writing to slave device, writer wakes up kworker to flush
> data on tty_port to reader, and the kworker finds that reader
> has no room to store data so room <= 0 is met. At this moment,
> reader consumes all the data on reader buffer and call
> n_tty_kick_worker to check ldata->no_room which is false and
> reader quits reading. Then kworker sets ldata->no_room=true
> and quits too.
> 
> If write buffer is not full, writer will wake kworker to flush data
> again after following writes, but if writer buffer is full and writer
> goes to sleep, kworker will never be woken again and tty device is
> blocked.
> 
> This problem can be solved with a check for read buffer size inside
> n_tty_receive_buf_common, if read buffer is empty and ldata->no_room
> is true, a call to n_tty_kick_worker is necessary to keep flushing
> data to reader.
> 
> Signed-off-by: cael <juanfengpy@xxxxxxxxx>
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> 
> ---
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index efc72104c840..21241ea7cdb9 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -201,8 +201,8 @@ static void n_tty_kick_worker(struct tty_struct *tty)
>         struct n_tty_data *ldata = tty->disc_data;
> 
>         /* Did the input worker stop? Restart it */
> -       if (unlikely(ldata->no_room)) {
> -               ldata->no_room = 0;
> +       if (unlikely(READ_ONCE(ldata->no_room))) {
> +               WRITE_ONCE(ldata->no_room, 0);
> 
>                 WARN_RATELIMIT(tty->port->itty == NULL,
>                                 "scheduling with invalid itty\n");
> @@ -1632,7 +1632,7 @@ n_tty_receive_buf_common(struct tty_struct *tty,
> const unsigned char *cp,
>                         if (overflow && room < 0)
>                                 ldata->read_head--;
>                         room = overflow;
> -                       ldata->no_room = flow && !room;
> +                       WRITE_ONCE(ldata->no_room, flow && !room);
>                 } else
>                         overflow = 0;
> 
> @@ -1663,6 +1663,21 @@ n_tty_receive_buf_common(struct tty_struct
> *tty, const unsigned char *cp,
>         } else
>                 n_tty_check_throttle(tty);
> 
> +       if (READ_ONCE(ldata->no_room)) {
> +               /*
> +                * Reader ensures that read_tail is updated before
> checking no_room,
> +                * make sure that no_room is set before reading read_tail here.
> +                * Now no_room is visible by reader, the race needs to
> be handled is
> +                * that reader has passed checkpoint for no_room and
> reader buffer
> +                * is empty, if so n_tty_kick_worker will not be
> called by reader,
> +                * instead, this function is called here.
> +                * barrier is paired with smp_mb() in n_tty_read()
> +                */
> +               smp_mb();
> +               if (!chars_in_buffer(tty))
> +                       n_tty_kick_worker(tty);
> +       }
> +
>         up_read(&tty->termios_rwsem);
> 
>         return rcvd;
> @@ -2180,8 +2195,14 @@ static ssize_t n_tty_read(struct tty_struct
> *tty, struct file *file,
>                 if (time)
>                         timeout = time;
>         }
> -       if (tail != ldata->read_tail)
> +       if (tail != ldata->read_tail) {
> +               /*
> +                * Make sure no_room is not read before setting read_tail,
> +                * paired with smp_mb() in n_tty_receive_buf_common()
> +                */
> 
> +               smp_mb();
>                 n_tty_kick_worker(tty);
> +       }
>         up_read(&tty->termios_rwsem);
> 
>         remove_wait_queue(&tty->read_wait, &wait);
> --
> 2.27.0

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
a patch that has triggered this response.  He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created.  Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.

You are receiving this message because of the following common error(s)
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If you wish to discuss this problem further, or you have questions about
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