linux-kernel - Re: [PATCH] leds: trigger: ledtrig-backlight: Fix invalid memory access in fb

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Date:	Mon, 26 Aug 2013 17:51:01 -0700
From:	Bryan Wu <cooloney@...il.com>
To:	Manfred Schlaegl <manfred.schlaegl@....at>
Cc:	Richard Purdie <rpurdie@...ys.net>,
	Jingoo Han <jg1.han@...sung.com>,
	"Milo(Woogyom) Kim" <milo.kim@...com>,
	Sachin Kamat <sachin.kamat@...aro.org>,
	Linux LED Subsystem <linux-leds@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] leds: trigger: ledtrig-backlight: Fix invalid memory
 access in fb_event notification callback

On Tue, Aug 13, 2013 at 4:17 AM, Manfred Schlaegl
<manfred.schlaegl@....at> wrote:
> 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

> Signed-off-by: Manfred Schlaegl <manfred.schlaegl@....at>
> ---
>
> 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.
>
>
> diff --git a/drivers/leds/trigger/ledtrig-backlight.c b/drivers/leds/trigger/ledtrig-backlight.c
> index 3c9c88a..2538dbe 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;
> +
> +       /* 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;
>  }
>
>
>
>
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