tty: Fix data race between tiocsti() and flush_to_ldisc()

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 waiting for the
ops->receive_buf() to finish its work, 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 we will lost the data.

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

Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
Reviewed-by: NJiri Slaby <jirislaby@kernel.org>
Signed-off-by: NNguyen Dinh Phi <phind.uet@gmail.com>
Link: https://lore.kernel.org/r/20210823000641.2082292-1-phind.uet@gmail.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -2290,8 +2290,6 @@ static int tty_fasync(int fd, struct file *filp, int on)
  *	Locking:
  *		Called functions take tty_ldiscs_lock
  *		current->signal->tty check is safe without locks
- *
- *	FIXME: may race normal receive processing
  */
 
 static int tiocsti(struct tty_struct *tty, char __user *p)
@@ -2307,8 +2305,10 @@ 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;
 }