fbdev: color map copying bounds checking

Copying color maps to userspace doesn't check the value of to->start,
which will cause kernel heap buffer OOB read due to signedness wraps.

CVE-2016-8405

Link: http://lkml.kernel.org/r/20170105224249.GA50925@beast
Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Peter Pi (@heisecode) of Trend Micro
Cc: Min Chong <mchong@google.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/fbdev/core/fbcmap.c

@@ -163,17 +163,18 @@ void fb_dealloc_cmap(struct fb_cmap *cmap)
 
 int fb_copy_cmap(const struct fb_cmap *from, struct fb_cmap *to)
 {
-	int tooff = 0, fromoff = 0;
-	int size;
+	unsigned int tooff = 0, fromoff = 0;
+	size_t size;
 
 	if (to->start > from->start)
 		fromoff = to->start - from->start;
 	else
 		tooff = from->start - to->start;
-	size = to->len - tooff;
-	if (size > (int) (from->len - fromoff))
-		size = from->len - fromoff;
-	if (size <= 0)
+	if (fromoff >= from->len || tooff >= to->len)
+		return -EINVAL;
+
+	size = min_t(size_t, to->len - tooff, from->len - fromoff);
+	if (size == 0)
 		return -EINVAL;
 	size *= sizeof(u16);
 
@@ -187,17 +188,18 @@ int fb_copy_cmap(const struct fb_cmap *from, struct fb_cmap *to)
 
 int fb_cmap_to_user(const struct fb_cmap *from, struct fb_cmap_user *to)
 {
-	int tooff = 0, fromoff = 0;
-	int size;
+	unsigned int tooff = 0, fromoff = 0;
+	size_t size;
 
 	if (to->start > from->start)
 		fromoff = to->start - from->start;
 	else
 		tooff = from->start - to->start;
-	size = to->len - tooff;
-	if (size > (int) (from->len - fromoff))
-		size = from->len - fromoff;
-	if (size <= 0)
+	if (fromoff >= from->len || tooff >= to->len)
+		return -EINVAL;
+
+	size = min_t(size_t, to->len - tooff, from->len - fromoff);
+	if (size == 0)
 		return -EINVAL;
 	size *= sizeof(u16);