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.
Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Jiri Slaby jslaby@suse.com Cc: stable@vger.kernel.org Signed-off-by: Dmitry Safonov dima@arista.com --- drivers/tty/tty_io.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index 5e5da9acaf0a..3ef8b977b167 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -1267,15 +1267,20 @@ static int tty_reopen(struct tty_struct *tty) if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN)) return -EBUSY;
- tty->count++; + retval = tty_ldisc_lock(tty, 5 * HZ); + if (retval) + return retval;
+ tty->count++; if (tty->ldisc) - return 0; + goto out_unlock;
retval = tty_ldisc_reinit(tty, tty->termios.c_line); if (retval) tty->count--;
+out_unlock: + tty_ldisc_unlock(tty); return retval; }