linux-kernel - [139/289] TTY: dont allow reopen when ldisc is changing

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Message-Id: <20101208005735.299793812@clark.site>
Date:	Tue, 07 Dec 2010 16:58:36 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Jiri Slaby <jslaby@...e.cz>, Kyle McMartin <kyle@...artin.ca>
Subject: [139/289] TTY: dont allow reopen when ldisc is changing

2.6.36-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Jiri Slaby <jslaby@...e.cz>

commit e2efafbf139d2bfdfe96f2901f03189fecd172e4 upstream.

There are many WARNINGs like the following reported nowadays:
WARNING: at drivers/tty/tty_io.c:1331 tty_open+0x2a2/0x49a()
Hardware name: Latitude E6500
Modules linked in:
Pid: 1207, comm: plymouthd Not tainted 2.6.37-rc3-mmotm1123 #3
Call Trace:
 [<ffffffff8103b189>] warn_slowpath_common+0x80/0x98
 [<ffffffff8103b1b6>] warn_slowpath_null+0x15/0x17
 [<ffffffff8128a3ab>] tty_open+0x2a2/0x49a
 [<ffffffff810fd53f>] chrdev_open+0x11d/0x146
...

This means tty_reopen is called without TTY_LDISC set. For further
considerations, note tty_lock is held in tty_open. TTY_LDISC is cleared in:
1) __tty_hangup from tty_ldisc_hangup to tty_ldisc_enable. During this
section tty_lock is held. However tty_lock is temporarily dropped in
the middle of the function by tty_ldisc_hangup.

2) tty_release via tty_ldisc_release till the end of tty existence. If
tty->count <= 1, tty_lock is taken, TTY_CLOSING bit set and then
tty_ldisc_release called. tty_reopen checks TTY_CLOSING before checking
TTY_LDISC.

3) tty_set_ldisc from tty_ldisc_halt to tty_ldisc_enable. We:
   * take tty_lock, set TTY_LDISC_CHANGING, put tty_lock
   * call tty_ldisc_halt (clear TTY_LDISC), tty_lock is _not_ held
   * do some other work
   * take tty_lock, call tty_ldisc_enable (set TTY_LDISC), put
     tty_lock

I cannot see how 2) can be a problem, as there I see no race. OTOH, 1)
and 3) can happen without problems. This patch the case 3) by checking
TTY_LDISC_CHANGING along with TTY_CLOSING in tty_reopen. 1) will be
fixed in the following patch.

Nicely reproducible with two processes:
while (1) {
	fd = open("/dev/ttyS1", O_RDWR);
	if (fd < 0) {
		warn("open");
		continue;
	}
	close(fd);
}
--------
while (1) {
        fd = open("/dev/ttyS1", O_RDWR);
        ld1 = 0; ld2 = 2;
        while (1) {
                ioctl(fd, TIOCSETD, &ld1);
                ioctl(fd, TIOCSETD, &ld2);
        }
        close(fd);
}

Signed-off-by: Jiri Slaby <jslaby@...e.cz>
Reported-by: <Valdis.Kletnieks@...edu>
Cc: Kyle McMartin <kyle@...artin.ca>
Cc: Alan Cox <alan@...rguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 drivers/char/tty_io.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1304,7 +1304,8 @@ static int tty_reopen(struct tty_struct
 {
 	struct tty_driver *driver = tty->driver;

-	if (test_bit(TTY_CLOSING, &tty->flags))
+	if (test_bit(TTY_CLOSING, &tty->flags) ||
+			test_bit(TTY_LDISC_CHANGING, &tty->flags))
 		return -EIO;

 	if (driver->type == TTY_DRIVER_TYPE_PTY &&

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