Re: tty: fix a possible hang on tty device

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jun 06, 2022 at 09:40:16PM +0800, cael wrote:
> From: cael <juanfengpy@gmail.com>
> Subject:[PATCH v3] 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@gmail.com>
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index efc72104c840..544f782b9a11 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 +1663,24 @@ n_tty_receive_buf_common(struct tty_struct
> *tty, const unsigned char *cp,
>         } else
>                 n_tty_check_throttle(tty);
> 
> +       if (unlikely(ldata->no_room)) {
> +               /*
> +                * Barrier here is to ensure to read the latest read_tail in
> +                * chars_in_buffer() and to make sure that read_tail
> is not loaded
> +                * before ldata->no_room is set, otherwise, following
> race may occur:
> +                * n_tty_receive_buf_common() |n_tty_read()
> +                * chars_in_buffer() > 0      |
> +                *
> |copy_from_read_buf()->chars_in_buffer()==0
> +                *                            |if (ldata->no_room)
> +                * ldata->no_room = 1         |
> +                * Then both kworker and reader will fail to kick
> n_tty_kick_worker(),
> +                * smp_mb 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 +2198,23 @@ 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,
> +                * otherwise, following race may occur:
> +                * n_tty_read()
> |n_tty_receive_buf_common()
> +                * if(ldata->no_room)->false            |
> +                *                                      |ldata->no_room = 1
> +                *                                      |char_in_buffer() > 0
> +                * ldata->read_tail = ldata->commit_head|
> +                * Then copy_from_read_buf() in reader consumes all the data
> +                * in read buffer, both reader and kworker will fail to kick
> +                * tty_buffer_restart_work().
> +                * smp_mb is 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


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