tty: Fix a USB serial crash/scribble

The port lock is used to protect the port state. However the port structure
is freed on a hangup, then the lock taken on a close. The right fix is to
drop the port on tty->shutdown() but we can't yet do that due to sleep v
non-sleeping rules. Instead do the next best thing and fix it up when we are
not in -rc season.

Reported-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Tested-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 1 deletions

diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index bd7581b3a48a..3c8923f62ed1 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -340,6 +340,22 @@ static void serial_close(struct tty_struct *tty, struct file *filp)
 
         dbg("%s - port %d", __func__, port->number);
 
+        /* FIXME:
+           This leaves a very narrow race. Really we should do the
+           serial_do_free() on tty->shutdown(), but tty->shutdown can
+           be called from IRQ context and serial_do_free can sleep.
+
+           The right fix is probably to make the tty free (which is rare)
+           and thus tty->shutdown() occur via a work queue and simplify all
+           the drivers that use it.
+        */
+        if (tty_hung_up_p(filp)) {
+                /* serial_hangup already called serial_down at this point.
+                   Another user may have already reopened the port but
+                   serial_do_free is refcounted */
+                serial_do_free(port);
+                return;
+        }
 
         if (tty_port_close_start(&port->port, tty, filp) == 0)
                 return;
@@ -355,7 +371,8 @@ static void serial_hangup(struct tty_struct *tty)
         struct usb_serial_port *port = tty->driver_data;
         serial_do_down(port);
         tty_port_hangup(&port->port);
-        serial_do_free(port);
+        /* We must not free port yet - the USB serial layer depends on it's
+           continued existence */
 }
 
 static int serial_write(struct tty_struct *tty, const unsigned char *buf,