drm: Validate requested virtual size against allocated fb size

mplayer -vo fbdev tries to create a screen that is twice as tall as the
allocated framebuffer for "doublebuffering". By default, and all in-tree
users, only sufficient memory is allocated and mapped to satisfy the
smallest framebuffer and the virtual size is no larger than the actual.
For these users, we should therefore reject any userspace request to
create a screen that requires a buffer larger than the framebuffer
originally allocated.

References: https://bugs.freedesktop.org/show_bug.cgi?id=38138
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -559,9 +559,13 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
 		return -EINVAL;
 
 	/* Need to resize the fb object !!! */
-	if (var->bits_per_pixel > fb->bits_per_pixel || var->xres > fb->width || var->yres > fb->height) {
+	if (var->bits_per_pixel > fb->bits_per_pixel ||
+	    var->xres > fb->width || var->yres > fb->height ||
+	    var->xres_virtual > fb->width || var->yres_virtual > fb->height) {
 		DRM_DEBUG("fb userspace requested width/height/bpp is greater than current fb "
-			  "object %dx%d-%d > %dx%d-%d\n", var->xres, var->yres, var->bits_per_pixel,
+			  "request %dx%d-%d (virtual %dx%d) > %dx%d-%d\n",
+			  var->xres, var->yres, var->bits_per_pixel,
+			  var->xres_virtual, var->yres_virtual,
 			  fb->width, fb->height, fb->bits_per_pixel);
 		return -EINVAL;
 	}