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SubjectRe: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
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On 08/29/2018, 04:40 PM, Jiri Slaby wrote:
> On 08/29/2018, 04:23 AM, 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.

...

> So what about:
> tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
> if (!tty->ldisc)
> ret = tty_ldisc_reinit(tty, tty->termios.c_line);
> tty_ldisc_unlock(tty);
>
> if (!ret)
> tty->count++;
>
> return ret;

I forgot to add that I debugged a different NULL ptr deref to this very
same root cause today (set_termios called with NULL tty->disc_data). So
really, tty_reinit's ldisc change must be protected by the ldisc_sem,
otherwise other threads will see tty->ldisc, but not tty->disc_data.

thanks,
--
js
suse labs

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