Hi,
On Thu, Nov 01, 2018 at 12:24:47AM +0000, Dmitry Safonov wrote:
> tty_ldisc_reinit() doesn't race with neither tty_ldisc_hangup()
> nor set_ldisc() nor tty_ldisc_release() as they use tty lock.
> But it races with anyone who expects line discipline to be the same
> after hoding read semaphore in tty_ldisc_ref().
>
> We've seen the following crash on v4.9.108 stable:
>
> BUG: unable to handle kernel paging request at 0000000000002260
> IP: [..] n_tty_receive_buf_common+0x5f/0x86d
> Workqueue: events_unbound flush_to_ldisc
> Call Trace:
> [..] n_tty_receive_buf2
> [..] tty_ldisc_receive_buf
> [..] flush_to_ldisc
> [..] process_one_work
> [..] worker_thread
> [..] kthread
> [..] ret_from_fork
>
> tty_ldisc_reinit() should be called with ldisc_sem hold for writing,
> which will protect any reader against line discipline changes.
>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Jiri Slaby <jslaby@xxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # b027e2298bd5 ("tty: fix data race between tty_init_dev and flush of buf")
> Reviewed-by: Jiri Slaby <jslaby@xxxxxxx>
> Reported-by: syzbot+3aa9784721dfb90e984d@xxxxxxxxxxxxxxxxxxxxxxxxx
> Tested-by: Mark Rutland <mark.rutland@xxxxxxx>
> Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Feel free to add
Tested-by: Tycho Andersen <tycho@xxxxxxxx>
to this as well. We've recently seen this bug (well, the one that
syzbot reported), and this patch fixes it.
Tycho
