linux-kernel - [PATCH v2 1/2] TTY: don't allow reopen when ldisc is changing

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 29 Nov 2010 10:16:53 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	gregkh@...e.de
Cc:	linux-kernel@...r.kernel.org, jirislaby@...il.com,
	Kyle McMartin <kyle@...artin.ca>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: [PATCH v2 1/2] TTY: don't allow reopen when ldisc is changing

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>
---
 drivers/tty/tty_io.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index c05c5af..878f6d6 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1310,7 +1310,8 @@ static int tty_reopen(struct tty_struct *tty)
 {
 	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 &&
-- 
1.7.3.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/