On 08/29/2018, 05:19 PM, Tetsuo Handa wrote:
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=f08670354701fa64cc0dd3c0128a491bdb16adc... .
syzbot is now testing a patch from Jiri Slaby.
Yes, my patch passed, so could you add: Reported-by: syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com
(not adding tested-by as this particular patch was not tested, but shoiuld work the same way.)
thanks,