leds: trigger: ledtrig-backlight: Fix invalid memory access in fb_event notification callback

fb_notifier_callback is called on any event fired by
fb_notifier_call_chain. Events may, or may not contain some data
(fb_event.data). In case of FB_EVENT_BLANK fb_event.data contains a
pointer to an integer holdingthe blank state. The Problem is, that in
ledtrig-backlight.c - fb_notifier_callback the pointer to blank state
is dereferenced BEFORE the event-type is checked.

Obviously this leads to problems with other events than FB_EVENT_BLANK,
where fb_event.data is undefined or NULL. It seems, that this problem
existed ever since the driver was added.

Like in drivers/video/backlight/backlight.c line 43 I would suggest to
return immediately on events other than FB_EVENT_BLANK.

Signed-off-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

1 files changed, 16 insertions, 14 deletions

diff --git a/drivers/leds/trigger/ledtrig-backlight.c b/drivers/leds/trigger/ledtrig-backlight.c
index 3c9c88a07eb8..47e55aa9eefa 100644
--- a/drivers/leds/trigger/ledtrig-backlight.c
+++ b/drivers/leds/trigger/ledtrig-backlight.c
@@ -36,26 +36,28 @@ static int fb_notifier_callback(struct notifier_block *p,
                                         struct bl_trig_notifier, notifier);
         struct led_classdev *led = n->led;
         struct fb_event *fb_event = data;
-        int *blank = fb_event->data;
-        int new_status = *blank ? BLANK : UNBLANK;
+        int *blank;
+        int new_status;
 
-        switch (event) {
-        case FB_EVENT_BLANK:
-                if (new_status == n->old_status)
-                        break;
+        /* If we aren't interested in this event, skip it immediately ... */
+        if (event != FB_EVENT_BLANK)
+                return 0;
 
-                if ((n->old_status == UNBLANK) ^ n->invert) {
-                        n->brightness = led->brightness;
-                        __led_set_brightness(led, LED_OFF);
-                } else {
-                        __led_set_brightness(led, n->brightness);
-                }
+        blank = fb_event->data;
+        new_status = *blank ? BLANK : UNBLANK;
 
-                n->old_status = new_status;
+        if (new_status == n->old_status)
+                return 0;
 
-                break;
+        if ((n->old_status == UNBLANK) ^ n->invert) {
+                n->brightness = led->brightness;
+                __led_set_brightness(led, LED_OFF);
+        } else {
+                __led_set_brightness(led, n->brightness);
         }
 
+        n->old_status = new_status;
+
         return 0;
 }