[PATCH] usb: appledisplay: fix use-after-free in bl_get_brightness

[Phong Tran]

In context of USB disconnect, the delaywork trigger and calling
appledisplay_bl_get_brightness() and the msgdata was freed.

add the checking return value of usb_control_msg() and only update the
data while the retval is valid.

Reported-by: syzbot+495dab1f175edc9c2f13@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-and-tested-by:
syzbot+495dab1f175edc9c2f13@xxxxxxxxxxxxxxxxxxxxxxxxx

https://groups.google.com/d/msg/syzkaller-bugs/dRmkh2UYusY/l2a6Mg3FAQAJ

Signed-off-by: Phong Tran <tranmanphong@xxxxxxxxx>
---
 drivers/usb/misc/appledisplay.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/misc/appledisplay.c b/drivers/usb/misc/appledisplay.c
index ac92725458b5..3e3dfa5a3954 100644
--- a/drivers/usb/misc/appledisplay.c
+++ b/drivers/usb/misc/appledisplay.c
@@ -164,7 +164,8 @@ static int appledisplay_bl_get_brightness(struct backlight_device *bd)
 		0,
 		pdata->msgdata, 2,
 		ACD_USB_TIMEOUT);
-	brightness = pdata->msgdata[1];
+	if (retval >= 0)
+		brightness = pdata->msgdata[1];
 	mutex_unlock(&pdata->sysfslock);
 
 	if (retval < 0)
-- 
2.20.1