patch "tty: fix port buffer locking" added to tty-linus

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    tty: fix port buffer locking

to my tty git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the
next -rc kernel release.

If you have any questions about this process, please let me know.


>From 925bb1ce47f429f69aad35876df7ecd8c53deb7e Mon Sep 17 00:00:00 2001
From: Vegard Nossum <vegard.nossum@oracle.com>
Date: Thu, 11 May 2017 12:18:52 +0200
Subject: tty: fix port buffer locking

tty_insert_flip_string_fixed_flag() is racy against itself when called
from the ioctl(TCXONC, TCION/TCIOFF) path [1] and the flush_to_ldisc()
workqueue path [2].

The problem is that port->buf.tail->used is modified without consistent
locking; the ioctl path takes tty->atomic_write_lock, whereas the workqueue
path takes ldata->output_lock.

We cannot simply take ldata->output_lock, since that is specific to the
N_TTY line discipline.

It might seem natural to try to take port->buf.lock inside
tty_insert_flip_string_fixed_flag() and friends (where port->buf is
actually used/modified), but this creates problems for flush_to_ldisc()
which takes it before grabbing tty->ldisc_sem, o_tty->termios_rwsem,
and ldata->output_lock.

Therefore, the simplest solution for now seems to be to take
tty->atomic_write_lock inside tty_port_default_receive_buf(). This lock
is also used in the write path [3] with a consistent ordering.

[1]: Call Trace:
 tty_insert_flip_string_fixed_flag
 pty_write
 tty_send_xchar                     // down_read(&o_tty->termios_rwsem)
                                    // mutex_lock(&tty->atomic_write_lock)
 n_tty_ioctl_helper
 n_tty_ioctl
 tty_ioctl                          // down_read(&tty->ldisc_sem)
 do_vfs_ioctl
 SyS_ioctl

[2]: Workqueue: events_unbound flush_to_ldisc
Call Trace:
 tty_insert_flip_string_fixed_flag
 pty_write
 tty_put_char
 __process_echoes
 commit_echoes                      // mutex_lock(&ldata->output_lock)
 n_tty_receive_buf_common
 n_tty_receive_buf2
 tty_ldisc_receive_buf              // down_read(&o_tty->termios_rwsem)
 tty_port_default_receive_buf       // down_read(&tty->ldisc_sem)
 flush_to_ldisc                     // mutex_lock(&port->buf.lock)
 process_one_work

[3]: Call Trace:
 tty_insert_flip_string_fixed_flag
 pty_write
 n_tty_write                        // mutex_lock(&ldata->output_lock)
                                    // down_read(&tty->termios_rwsem)
 do_tty_write (inline)              // mutex_lock(&tty->atomic_write_lock)
 tty_write                          // down_read(&tty->ldisc_sem)
 __vfs_write
 vfs_write
 SyS_write

The bug can result in about a dozen different crashes depending on what
exactly gets corrupted when port->buf.tail->used points outside the
buffer.

The patch passes my LOCKDEP/PROVE_LOCKING testing but more testing is
always welcome.

Found using syzkaller.

Cc: <stable@vger.kernel.org>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/tty_port.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 0c880f17d27e..88dac3b79369 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -33,7 +33,9 @@ static int tty_port_default_receive_buf(struct tty_port *port,
 	if (!disc)
 		return 0;
 
+	mutex_lock(&tty->atomic_write_lock);
 	ret = tty_ldisc_receive_buf(disc, p, (char *)f, count);
+	mutex_unlock(&tty->atomic_write_lock);
 
 	tty_ldisc_deref(disc);
 
-- 
2.13.0