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 holding the 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 <[email protected]>

---

Background information:
I'm currently working on a IMX53(ARM) based hardware and Linux 3.11-rc5 and detected a problem in drivers/leds/trigger/ledtrig-backlight.c
ledtrig is used on our hardware (besides pwm-backlight) to switch off a gpio-line connected to display-supply enable-line, when the display is blanked.
On first boot after configuring ledtrig I detected an invalid memory access in ledtrig-backlight.c - fb_notifier_callback. In my special case the segfault was caused by an FB_EVENT_FB_REGISTERED, fired with undefined fb_event.data in drivers/video/fbmem.c line 1653.

drivers/leds/trigger/ledtrig-backlight.c +17 -15

index 3c9c88a..2538dbe 100644
@@ -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;
+
+       /* If we aren't interested in this event, skip it immediately ... */
+       if (event != FB_EVENT_BLANK)
+               return 0;
 
-       switch (event) {
-       case FB_EVENT_BLANK:
-               if (new_status == n->old_status)
-                       break;
+       blank = fb_event->data;
+       new_status = *blank ? BLANK : UNBLANK;
 
-               if ((n->old_status == UNBLANK) ^ n->invert) {
-                       n->brightness = led->brightness;
-                       __led_set_brightness(led, LED_OFF);
-               } else {
-                       __led_set_brightness(led, n->brightness);
-               }
+       if (new_status == n->old_status)
+               return 0;
 
-               n->old_status = new_status;
-
-               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;
 }
 

Quoted 8 lines · Manfred Schlaegl

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 holding the 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.

Indeed, this is an issue. Thanks for this nice catch and fixing.

I merged this patch into my tree and it will hit 3.11 release.

Thanks,
-Bryan

Quoted 52 lines

Signed-off-by Manfred Schlaegl <[email protected]>

---

Background information:
I'm currently working on a IMX53(ARM) based hardware and Linux 3.11-rc5 and detected a problem in drivers/leds/trigger/ledtrig-backlight.c
ledtrig is used on our hardware (besides pwm-backlight) to switch off a gpio-line connected to display-supply enable-line, when the display is blanked.
On first boot after configuring ledtrig I detected an invalid memory access in ledtrig-backlight.c - fb_notifier_callback. In my special case the segfault was caused by an FB_EVENT_FB_REGISTERED, fired with undefined fb_event.data in drivers/video/fbmem.c line 1653.

drivers/leds/trigger/ledtrig-backlight.c +12 -15

index 3c9c88a..2538dbe 100644
@@ -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;
+
+       /* If we aren't interested in this event, skip it immediately ... */
+       if (event != FB_EVENT_BLANK)
+               return 0;
 
-       switch (event) {
-       case FB_EVENT_BLANK:
-               if (new_status == n->old_status)
-                       break;
+       blank = fb_event->data;
+       new_status = *blank ? BLANK : UNBLANK;
 
-               if ((n->old_status == UNBLANK) ^ n->invert) {
-                       n->brightness = led->brightness;
-                       __led_set_brightness(led, LED_OFF);
-               } else {
-                       __led_set_brightness(led, n->brightness);
-               }
+       if (new_status == n->old_status)
+               return 0;
 
-               n->old_status = new_status;
-
-               break;
+       if ((n->old_status == UNBLANK) ^ n->invert) {
+               n->brightness = led->brightness;

drivers/leds/trigger/ledtrig-backlight.c static int fb_notifier_callback(struct notifier_block *p, · line 63

Quoted 10 lines

drivers/leds/trigger/ledtrig-backlight.c +5 -0

+               __led_set_brightness(led, LED_OFF);
+       } else {
+               __led_set_brightness(led, n->brightness);
        }
 
+       n->old_status = new_status;
+
        return 0;
 }
 

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