On 2018/08/29 11:23, 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 > > I think, tty_ldisc_reinit() should be called with ldisc_sem hold for > writing, which will protect any reader against line discipline changes. > > Note: I failed to reproduce the described crash, so obiviously can't > guarantee that this is the place where line discipline was switched. This will be same with a report at https://syzkaller.appspot.com/bug?id=f08670354701fa64cc0dd3c0128a491bdb16adcc . syzbot is now testing a patch from Jiri Slaby.
