tty: Hold tty_ldisc_lock() during tty_reopen()

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: Jiri Slaby <[email protected]>
Cc: [email protected] # b027e2298bd5 ("tty: fix data race between tty_init_dev and flush of buf")
Reviewed-by: Jiri Slaby <[email protected]>
Reported-by: [email protected]
Tested-by: Mark Rutland <[email protected]>
Tested-by: Tetsuo Handa <[email protected]>
Signed-off-by: Dmitry Safonov <[email protected]>
Tested-by: Tycho Andersen <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

0 files changed, 0 insertions, 0 deletions