[PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Nguyen Dinh Phi <phind.uet@gmail.com>]

The ops->receive_buf() may be accessed concurrently from these two
functions.  If the driver flushes data to the line discipline
receive_buf() method while tiocsti() is using the ops->receive_buf(),
the data race will happen.

For example:
tty_ioctl                       |tty_ldisc_receive_buf
 ->tioctsi                      | ->tty_port_default_receive_buf
                                |  ->tty_ldisc_receive_buf
   ->hci_uart_tty_receive       |   ->hci_uart_tty_receive
    ->h4_recv                   |    ->h4_recv

In this case, the h4 receive buffer will be overwritten by the
latecomer, and it cause a memory leak.

Hence, change tioctsi() function to use the exclusive lock interface
from tty_buffer to avoid the data race.

Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
---
 drivers/tty/tty_io.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e8532006e960..746fe13a2054 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
 	ld = tty_ldisc_ref_wait(tty);
 	if (!ld)
 		return -EIO;
+	tty_buffer_lock_exclusive(tty->port);
 	if (ld->ops->receive_buf)
 		ld->ops->receive_buf(tty, &ch, &mbz, 1);
+	tty_buffer_unlock_exclusive(tty->port);
 	tty_ldisc_deref(ld);
 	return 0;
 }
--
2.25.1