linux-kernel - [PATCHv2 2/4] tty: Hold tty_ldisc_lock() during tty

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Message-Id: <20180903165257.29227-3-dima@arista.com>
Date:   Mon,  3 Sep 2018 17:52:55 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Nathan March <nathan@...net>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, stable@...r.kernel.org
Subject: [PATCHv2 2/4] 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: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jiri Slaby <jslaby@...e.com>
Cc: stable@...r.kernel.org # depends on commit b027e2298bd5 ("tty: fix
data race between tty_init_dev and flush of buf")
Reported-by: syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com
Tested-by: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Signed-off-by: Dmitry Safonov <dima@...sta.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;
 }
 
-- 
2.13.6