From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759582AbcINJgy (ORCPT ); Wed, 14 Sep 2016 05:36:54 -0400 Received: from foss.arm.com ([217.140.101.70]:36222 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753197AbcINJgw (ORCPT ); Wed, 14 Sep 2016 05:36:52 -0400 Date: Wed, 14 Sep 2016 10:36:34 +0100 From: Mark Rutland To: kernel-hardening@lists.openwall.com Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-x86_64@vger.kernel.org, juerg.haefliger@hpe.com, vpk@cs.columbia.edu Subject: Re: [kernel-hardening] [RFC PATCH v2 0/3] Add support for eXclusive Page Frame Ownership (XPFO) Message-ID: <20160914093634.GB13121@leverpostej> References: <20160902113909.32631-1-juerg.haefliger@hpe.com> <20160914071901.8127-1-juerg.haefliger@hpe.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160914071901.8127-1-juerg.haefliger@hpe.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Wed, Sep 14, 2016 at 09:18:58AM +0200, Juerg Haefliger wrote: > This patch series adds support for XPFO which protects against 'ret2dir' > kernel attacks. The basic idea is to enforce exclusive ownership of page > frames by either the kernel or userspace, unless explicitly requested by > the kernel. Whenever a page destined for userspace is allocated, it is > unmapped from physmap (the kernel's page table). When such a page is > reclaimed from userspace, it is mapped back to physmap. > Known issues/limitations: > - Only supports x86-64 (for now) > - Only supports 4k pages (for now) > - There are most likely some legitimate uses cases where the kernel needs > to access userspace which need to be made XPFO-aware > - Performance penalty > > Reference paper by the original patch authors: > http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf Just to check, doesn't DEBUG_RODATA ensure that the linear mapping is non-executable on x86_64 (as it does for arm64)? For both arm64 and x86_64, DEBUG_RODATA is mandatory (or soon to be so). Assuming that implies a lack of execute permission for x86_64, that should provide a similar level of protection against erroneously branching to addresses in the linear map, without the complexity and overhead of mapping/unmapping pages. So to me it looks like this approach may only be useful for architectures without page-granular execute permission controls. Is this also intended to protect against erroneous *data* accesses to the linear map? Am I missing something? Thanks, Mark.