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Subject[PATCH] tty: fix port buffer locking
Date
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>
---
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 1d21a9c1d33e..ef4dd596b864 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -34,7 +34,9 @@ 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);

--
2.12.0.rc0
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