fb: fix fb_pan_display range check

Fix the range check for panning.  The current code fails to detect some
invalid values (very high ones that can occur if an app tries to move
further up/left than 0,0) as the check uses the unknown values for
calculation so that an overflow can occur.

To fix this it is sufficient to move the calculation to the right side to
use only trusted values.

Kai Jiang detected this problem and proposed an initial patch.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Kai Jiang <b18973@freescale.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
index a85c818be945..346f257215a7 100644
--- a/drivers/video/fbmem.c
+++ b/drivers/video/fbmem.c
@@ -871,8 +871,8 @@ fb_pan_display(struct fb_info *info, struct fb_var_screeninfo *var)
                 err = -EINVAL;
 
         if (err || !info->fbops->fb_pan_display ||
-            var->yoffset + yres > info->var.yres_virtual ||
-            var->xoffset + info->var.xres > info->var.xres_virtual)
+            var->yoffset > info->var.yres_virtual - yres ||
+            var->xoffset > info->var.xres_virtual - info->var.xres)
                 return -EINVAL;
 
         if ((err = info->fbops->fb_pan_display(var, info)))