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From: Dmitry Safonov <dima@arista.com>
To: linux-kernel@vger.kernel.org
Cc: "Dmitry Safonov" <0x7f454c46@gmail.com>,
	"Dmitry Safonov" <dima@arista.com>,
	"Daniel Axtens" <dja@axtens.net>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Michael Neuling" <mikey@neuling.org>,
	"Mikulas Patocka" <mpatocka@redhat.com>,
	"Pasi Kärkkäinen" <pasik@iki.fi>,
	"Peter Hurley" <peter@hurleysoftware.com>,
	"Sergey Senozhatsky" <sergey.senozhatsky.work@gmail.com>,
	"Tan Xiaojun" <tanxiaojun@huawei.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Jiri Slaby" <jslaby@suse.com>,
	stable@vger.kernel.org
Subject: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
Date: Wed, 29 Aug 2018 03:23:51 +0100	[thread overview]
Message-ID: <20180829022353.23568-3-dima@arista.com> (raw)
In-Reply-To: <20180829022353.23568-1-dima@arista.com>

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;
 }
 
-- 
2.13.6


  parent reply	other threads:[~2018-08-29  2:24 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-29  2:23 [PATCH 0/4] tty: Hold write ldisc sem in tty_reopen() Dmitry Safonov
2018-08-29  2:23 ` [PATCH 1/4] tty: Drop tty->count on tty_reopen() failure Dmitry Safonov
2018-08-29 14:38   ` Jiri Slaby
2018-08-29 16:13     ` Dmitry Safonov
2018-08-31  6:47       ` Jiri Slaby
2018-08-31 11:54         ` Dmitry Safonov
2018-08-29  2:23 ` Dmitry Safonov [this message]
2018-08-29  4:34   ` [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen() Sergey Senozhatsky
2018-08-29 14:30     ` Dmitry Safonov
2018-08-30  5:16     ` Benjamin Herrenschmidt
2018-08-29 14:40   ` Jiri Slaby
2018-08-29 14:45     ` Jiri Slaby
2018-08-29 16:36     ` Dmitry Safonov
2018-08-29 15:19   ` Tetsuo Handa
2018-08-31  6:51     ` Jiri Slaby
2018-08-31 11:17       ` Tetsuo Handa
2018-08-31 11:21         ` Jiri Slaby
2018-08-31 12:12           ` Dmitry Safonov
2018-09-07  4:50   ` [LKP] [tty] 0b4f83d510: INFO:task_blocked_for_more_than#seconds kernel test robot
2018-09-07  6:39     ` Jiri Slaby
2018-09-07 11:12       ` Dmitry Safonov
2018-09-10  5:14       ` Sergey Senozhatsky
2018-09-10 18:50         ` Dmitry Safonov
2018-08-29  2:23 ` [PATCH 3/4] tty: Lock tty pair in tty_init_dev() Dmitry Safonov
2018-08-29 14:46   ` Jiri Slaby
2018-08-29 16:28     ` Dmitry Safonov
2018-08-31  6:54       ` Jiri Slaby
2018-08-31 12:22         ` Dmitry Safonov
2018-08-29  2:23 ` [PATCH 4/4] tty/lockdep: Add ldisc_sem asserts Dmitry Safonov
2018-08-30  7:03 ` [PATCH 0/4] tty: Hold write ldisc sem in tty_reopen() Pasi Kärkkäinen

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