drm: udl: Properly check framebuffer mmap offsets

The memmap options sent to the udl framebuffer driver were not being
checked for all sets of possible crazy values.  Fix this up by properly
bounding the allowed values.
Reported-by: Eyal Itkin <eyalit@checkpoint.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321154553.GA18454@kroah.com

drivers/gpu/drm/udl/udl_fb.c

@@ -159,10 +159,15 @@ static int udl_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)
 {
 	unsigned long start = vma->vm_start;
 	unsigned long size = vma->vm_end - vma->vm_start;
-	unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
+	unsigned long offset;
 	unsigned long page, pos;
 
-	if (offset + size > info->fix.smem_len)
+	if (vma->vm_pgoff > (~0UL >> PAGE_SHIFT))
+		return -EINVAL;
+
+	offset = vma->vm_pgoff << PAGE_SHIFT;
+
+	if (offset > info->fix.smem_len || size > info->fix.smem_len - offset)
 		return -EINVAL;
 
 	pos = (unsigned long)info->fix.smem_start + offset;