Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932705AbcJETeT (ORCPT ); Wed, 5 Oct 2016 15:34:19 -0400 Received: from mail-wm0-f42.google.com ([74.125.82.42]:38582 "EHLO mail-wm0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932658AbcJETeJ (ORCPT ); Wed, 5 Oct 2016 15:34:09 -0400 MIME-Version: 1.0 In-Reply-To: <1475690686-16138-1-git-send-email-william.c.roberts@intel.com> References: <1475690686-16138-1-git-send-email-william.c.roberts@intel.com> From: Kees Cook Date: Wed, 5 Oct 2016 12:34:07 -0700 X-Google-Sender-Auth: V2eqoPFctROCt2ilwox0Ss9sXXc Message-ID: Subject: Re: [PATCH] printk: introduce kptr_restrict level 3 To: "Roberts, William C" Cc: "kernel-hardening@lists.openwall.com" , Jonathan Corbet , "linux-doc@vger.kernel.org" , LKML , Nick Desaulniers , Dave Weinstein Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1912 Lines: 48 On Wed, Oct 5, 2016 at 11:04 AM, wrote: > From: William Roberts > > Some out-of-tree modules do not use %pK and just use %p, as it's > the common C paradigm for printing pointers. Because of this, > kptr_restrict has no affect on the output and thus, no way to > contain the kernel address leak. Solving this is certainly a good idea -- I'm all for finding a solid solution. > Introduce kptr_restrict level 3 that causes the kernel to > treat %p as if it was %pK and thus always prints zeros. I'm worried that this could break kernel internals where %p is being used and not exposed to userspace. Maybe those situations don't exist... Regardless, I would rather do what Grsecurity has done in this area, and whitelist known-safe values instead. For example, they have %pP for approved pointers, and %pX for approved dereference_function_descriptor() output. Everything else is censored if it is a value in kernel memory and destined for a user-space memory buffer: if ((unsigned long)ptr > TASK_SIZE && *fmt != 'P' && *fmt != 'X' && *fmt != 'K' && is_usercopy_object(buf)) { printk(KERN_ALERT "grsec: kernel infoleak detected! Please report this log to spender@grsecurity.net.\n"); dump_stack(); ptr = NULL; } The "is_usercopy_object()" test is something we can add, which is testing for a new SLAB flag that is used to mark slab caches as either used by user-space or not, which is done also through whitelisting. (For more details on this, see: http://www.openwall.com/lists/kernel-hardening/2016/06/08/10) Would you have time/interest to add the slab flags and is_usercopy_object()? The hardened usercopy part of the slab whitelisting can be separate, since it likely needs a different usercopy interface to sanely integrate with upstream. -Kees -- Kees Cook Nexus Security