From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f65.google.com ([209.85.208.65]:40533 "EHLO mail-ed1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727803AbeICVOA (ORCPT ); Mon, 3 Sep 2018 17:14:00 -0400 Received: by mail-ed1-f65.google.com with SMTP id j62-v6so1236128edd.7 for ; Mon, 03 Sep 2018 09:53:03 -0700 (PDT) From: Dmitry Safonov To: linux-kernel@vger.kernel.org Cc: Dmitry Safonov <0x7f454c46@gmail.com>, Dmitry Safonov , Daniel Axtens , Dmitry Vyukov , Michael Neuling , Mikulas Patocka , Nathan March , =?UTF-8?q?Pasi=20K=C3=A4rkk=C3=A4inen?= , Peter Hurley , Sergey Senozhatsky , Tan Xiaojun , Tetsuo Handa , syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com, Tetsuo Handa , Greg Kroah-Hartman , Jiri Slaby , stable@vger.kernel.org Subject: [PATCHv2 2/4] tty: Hold tty_ldisc_lock() during tty_reopen() Date: Mon, 3 Sep 2018 17:52:55 +0100 Message-Id: <20180903165257.29227-3-dima@arista.com> In-Reply-To: <20180903165257.29227-1-dima@arista.com> References: <20180903165257.29227-1-dima@arista.com> Sender: stable-owner@vger.kernel.org List-ID: 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: Greg Kroah-Hartman Cc: Jiri Slaby Cc: stable@vger.kernel.org # depends on commit b027e2298bd5 ("tty: fix data race between tty_init_dev and flush of buf") Reported-by: syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com Tested-by: Tetsuo Handa Signed-off-by: Dmitry Safonov --- 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; } -- 2.13.6