[PATCH v2 2/2] usb: gadget: u_serial: Fix race condition in TTY wakeup

[Kuen-Han Tsai <khtsai@google.com>]

A race condition occurs when gs_start_io() calls either gs_start_rx() or
gs_start_tx(), as those functions briefly drop the port_lock for
usb_ep_queue(). This allows gs_close() and gserial_disconnect() to clear
port.tty and port_usb, respectively.

Use the null-safe TTY Port helper function to wake up TTY.

Example
  CPU1:			      CPU2:
  gserial_connect() // lock
  			      gs_close() // await lock
  gs_start_rx()     // unlock
  usb_ep_queue()
  			      gs_close() // lock, reset port.tty and unlock
  gs_start_rx()     // lock
  tty_wakeup()      // NPE

Fixes: 35f95fd7f234 ("TTY: usb/u_serial, use tty from tty_port")
Cc: stable@vger.kernel.org
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
---
v2:
- Move the example up to the changelog

Traces:
[   51.494375][  T278] ttyGS1: shutdown
[   51.494817][  T269] android_work: sent uevent USB_STATE=DISCONNECTED
[   52.115792][ T1508] usb: [dm_bind] generic ttyGS1: super speed IN/ep1in OUT/ep1out
[   52.516288][ T1026] android_work: sent uevent USB_STATE=CONNECTED
[   52.551667][ T1533] gserial_connect: start ttyGS1
[   52.565634][ T1533] [khtsai] enter gs_start_io, ttyGS1, port->port.tty=0000000046bd4060
[   52.565671][ T1533] [khtsai] gs_start_rx, unlock port ttyGS1
[   52.591552][ T1533] [khtsai] gs_start_rx, lock port ttyGS1
[   52.619901][ T1533] [khtsai] gs_start_rx, unlock port ttyGS1
[   52.638659][ T1325] [khtsai] gs_close, lock port ttyGS1
[   52.656842][ T1325] gs_close: ttyGS1 (0000000046bd4060,00000000be9750a5) ...
[   52.683005][ T1325] [khtsai] gs_close, clear ttyGS1
[   52.683007][ T1325] gs_close: ttyGS1 (0000000046bd4060,00000000be9750a5) done!
[   52.708643][ T1325] [khtsai] gs_close, unlock port ttyGS1
[   52.747592][ T1533] [khtsai] gs_start_rx, lock port ttyGS1
[   52.747616][ T1533] [khtsai] gs_start_io, ttyGS1, going to call tty_wakeup(), port->port.tty=0000000000000000
[   52.747629][ T1533] Unable to handle kernel NULL pointer dereference at virtual address 00000000000001f8
---
 drivers/usb/gadget/function/u_serial.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index c043bdc30d8a..540dc5ab96fc 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -295,8 +295,8 @@ __acquires(&port->port_lock)
 			break;
 	}

-	if (do_tty_wake && port->port.tty)
-		tty_wakeup(port->port.tty);
+	if (do_tty_wake)
+		tty_port_tty_wakeup(&port->port);
 	return status;
 }

@@ -574,7 +574,7 @@ static int gs_start_io(struct gs_port *port)
 		gs_start_tx(port);
 		/* Unblock any pending writes into our circular buffer, in case
 		 * we didn't in gs_start_tx() */
-		tty_wakeup(port->port.tty);
+		tty_port_tty_wakeup(&port->port);
 	} else {
 		/* Free reqs only if we are still connected */
 		if (port->port_usb) {
--
2.50.0.rc2.692.g299adb8693-goog