Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Aug 08, 2021 at 03:05:13AM +0800, Nguyen Dinh Phi wrote:
> The ops->receive_buf() may be accessed concurrently from these two
> functions.  If the driver flushes data to the line discipline
> receive_buf() method while tiocsti() is using the ops->receive_buf(),
> the data race will happen.
> 
> For example:
> tty_ioctl                       |tty_ldisc_receive_buf
>  ->tioctsi                      | ->tty_port_default_receive_buf
>                                 |  ->tty_ldisc_receive_buf
>    ->hci_uart_tty_receive       |   ->hci_uart_tty_receive
>     ->h4_recv                   |    ->h4_recv
> 
> In this case, the h4 receive buffer will be overwritten by the
> latecomer, and it cause a memory leak.

That looks to be a bug in the h4 code, if the receive_buf() call can not
be run at the same time, it should have a lock in it, right?

> Hence, change tioctsi() function to use the exclusive lock interface
> from tty_buffer to avoid the data race.

Where is the lock being grabbed from the other receive_buf() call path
to ensure that the lock is always needed here?

> 
> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
> Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
> ---
>  drivers/tty/tty_io.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index e8532006e960..746fe13a2054 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>  	ld = tty_ldisc_ref_wait(tty);
>  	if (!ld)
>  		return -EIO;
> +	tty_buffer_lock_exclusive(tty->port);
>  	if (ld->ops->receive_buf)
>  		ld->ops->receive_buf(tty, &ch, &mbz, 1);
> +	tty_buffer_unlock_exclusive(tty->port);

Did this fix the syzbot reported issue?

thanks,

greg k-h
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