fbmem: Check virtual screen sizes in fb_set_var()

Verify that the fbdev or drm driver correctly adjusted the virtual
screen sizes. On failure report the failing driver and reject the screen
size change.
Signed-off-by: NHelge Deller <deller@gmx.de>
Reviewed-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Cc: stable@vger.kernel.org # v5.4+

drivers/video/fbdev/core/fbmem.c

@@ -1017,6 +1017,16 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
 	if (ret)
 		return ret;
 
+	/* verify that virtual resolution >= physical resolution */
+	if (var->xres_virtual < var->xres ||
+	    var->yres_virtual < var->yres) {
+		pr_warn("WARNING: fbcon: Driver '%s' missed to adjust virtual screen size (%ux%u vs. %ux%u)\n",
+			info->fix.id,
+			var->xres_virtual, var->yres_virtual,
+			var->xres, var->yres);
+		return -EINVAL;
+	}
+
 	if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
 		return 0;