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: [email protected]
Reviewed-by: Jiri Slaby <[email protected]>
Signed-off-by: Nguyen Dinh Phi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: stable <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e8532006e960ef1f12328b8f1a6e967c178d349a..6616d4a0d41deb6953106a4de63f3e3678eef94e 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/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;
 }