mx3fb: fix NULL pointer dereference in screen blanking.

When blanking an already blanked framebuffer, a kernel NULL pointer
dereference occurred, because mx3fb driver handles all kinds of screen
blanking (normal, vsync suspend, powerdown) in the same way.
Certain programs (Xorg X11 server) first do a normal blank, followed by
a powerdown blank, which triggered the bug.

Add an additional safeguard and make sdc_disable_channel() safe against
multiple calls independent of other logic.

Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.at>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>

1 files changed, 13 insertions, 0 deletions

diff --git a/drivers/video/mx3fb.c b/drivers/video/mx3fb.c
index bd768588cf10..e3406ab31305 100644
--- a/drivers/video/mx3fb.c
+++ b/drivers/video/mx3fb.c
@@ -382,6 +382,9 @@ static void sdc_disable_channel(struct mx3fb_info *mx3_fbi)
         uint32_t enabled;
         unsigned long flags;
 
+        if (mx3_fbi->txd == NULL)
+                return;
+
         spin_lock_irqsave(&mx3fb->lock, flags);
 
         enabled = sdc_fb_uninit(mx3_fbi);
@@ -986,9 +989,19 @@ static void __blank(int blank, struct fb_info *fbi)
 {
         struct mx3fb_info *mx3_fbi = fbi->par;
         struct mx3fb_data *mx3fb = mx3_fbi->mx3fb;
+        int was_blank = mx3_fbi->blank;
 
         mx3_fbi->blank = blank;
 
+        /* Attention!
+         * Do not call sdc_disable_channel() for a channel that is disabled
+         * already! This will result in a kernel NULL pointer dereference
+         * (mx3_fbi->txd is NULL). Hide the fact, that all blank modes are
+         * handled equally by this driver.
+         */
+        if (blank > FB_BLANK_UNBLANK && was_blank > FB_BLANK_UNBLANK)
+                return;
+
         switch (blank) {
         case FB_BLANK_POWERDOWN:
         case FB_BLANK_VSYNC_SUSPEND: