On 08/29/2018, 05:19 PM, Tetsuo Handa wrote:

On 2018/08/29 11:23, Dmitry Safonov wrote:

quoted

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.

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,
-- 
js
suse labs