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.
Backport-first: b027e2298bd5 ("tty: fix data race between tty_init_dev and flush of buf") Cc: stable@vger.kernel.org
Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Jiri Slaby jslaby@suse.com Reviewed-by: Jiri Slaby jslaby@suse.cz Reported-by: syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com Tested-by: Tetsuo Handa penguin-kernel@i-love.sakura.ne.jp Signed-off-by: Dmitry Safonov dima@arista.com --- drivers/tty/tty_io.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index 5e5da9acaf0a..a947719b4626 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -1255,7 +1255,7 @@ static void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct * static int tty_reopen(struct tty_struct *tty) { struct tty_driver *driver = tty->driver; - int retval; + int retval = 0;
if (driver->type == TTY_DRIVER_TYPE_PTY && driver->subtype == PTY_TYPE_MASTER) @@ -1267,15 +1267,18 @@ static int tty_reopen(struct tty_struct *tty) if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN)) return -EBUSY;
- tty->count++; + tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
+ 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; }