From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CF48CC433E3 for ; Sat, 8 Aug 2020 22:17:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B5CED206B5 for ; Sat, 8 Aug 2020 22:17:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726197AbgHHWRw (ORCPT ); Sat, 8 Aug 2020 18:17:52 -0400 Received: from jabberwock.ucw.cz ([46.255.230.98]:37712 "EHLO jabberwock.ucw.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725779AbgHHWRw (ORCPT ); Sat, 8 Aug 2020 18:17:52 -0400 Received: by jabberwock.ucw.cz (Postfix, from userid 1017) id EAB141C0BDA; Sun, 9 Aug 2020 00:17:49 +0200 (CEST) Date: Sun, 9 Aug 2020 00:17:48 +0200 From: Pavel Machek To: "Madhavan T. Venkataraman" Cc: Mark Rutland , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, oleg@redhat.com, x86@kernel.org Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor Message-ID: <20200808221748.GA1020@bug> References: <20200728131050.24443-1-madvenka@linux.microsoft.com> <20200731180955.GC67415@C02TD0UTHF1T.local> <6236adf7-4bed-534e-0956-fddab4fd96b6@linux.microsoft.com> <20200804143018.GB7440@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi! > Thanks for the lively discussion. I have tried to answer some of the > comments below. > > There are options today, e.g. > > > > a) If the restriction is only per-alias, you can have distinct aliases > > where one is writable and another is executable, and you can make it > > hard to find the relationship between the two. > > > > b) If the restriction is only temporal, you can write instructions into > > an RW- buffer, transition the buffer to R--, verify the buffer > > contents, then transition it to --X. > > > > c) You can have two processes A and B where A generates instrucitons into > > a buffer that (only) B can execute (where B may be restricted from > > making syscalls like write, mprotect, etc). > > The general principle of the mitigation is W^X. I would argue that > the above options are violations of the W^X principle. If they are > allowed today, they must be fixed. And they will be. So, we cannot > rely on them. Would you mind describing your threat model? Because I believe you are using model different from everyone else. In particular, I don't believe b) is a problem or should be fixed. I'll add d), application mmaps a file(R--), and uses write syscall to change trampolines in it. > b) This is again a violation. The kernel should refuse to give execute > ???????? permission to a page that was writeable in the past and refuse to > ???????? give write permission to a page that was executable in the past. Why? Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html