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@xxxxxxxxxx>
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@xxxxxxxxxxxxxxx>
Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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