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=-13.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1,USER_IN_DEF_DKIM_WL 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 976D6C433E0 for ; Tue, 11 Aug 2020 12:41:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6B76B20756 for ; Tue, 11 Aug 2020 12:41:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="PFQNuX5m" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728622AbgHKMlS (ORCPT ); Tue, 11 Aug 2020 08:41:18 -0400 Received: from linux.microsoft.com ([13.77.154.182]:52328 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726829AbgHKMlR (ORCPT ); Tue, 11 Aug 2020 08:41:17 -0400 Received: from [192.168.254.32] (unknown [47.187.206.220]) by linux.microsoft.com (Postfix) with ESMTPSA id 7A2A120B4908; Tue, 11 Aug 2020 05:41:16 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 7A2A120B4908 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1597149677; bh=/01YYim8UON1WnW57/jtTiKZ9zMxYZQ09Mw9cbuIu4A=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=PFQNuX5mWMquO2HGENEzafqUyJUE7/be/jyBGj5BPS+LLzBuKDCp8oxu/gpMGiuzS 6nmE9EJ+VCIZo4ZznNZO0l1rypST2o/WGeVSn+KjlmzrdKh1od3CFRwpGX0UIvppY1 +3sGwRwS/5aecpkyVSZ45vXgzNz/7thzHMTdnYOE= Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor To: Pavel Machek 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 References: <20200728131050.24443-1-madvenka@linux.microsoft.com> <20200731180955.GC67415@C02TD0UTHF1T.local> <6236adf7-4bed-534e-0956-fddab4fd96b6@linux.microsoft.com> <20200804143018.GB7440@C02TD0UTHF1T.local> <20200808221748.GA1020@bug> From: "Madhavan T. Venkataraman" Message-ID: <6cca8eac-f767-b891-dc92-eaa7504a0e8b@linux.microsoft.com> Date: Tue, 11 Aug 2020 07:41:15 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200808221748.GA1020@bug> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/8/20 5:17 PM, Pavel Machek wrote: > 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. It is a problem because a kernel that implements W^X properly will not allow it. It has no idea what has been done in userland. It has no idea that the user has checked and verified the buffer contents after transitioning the page to R--. > > I'll add d), application mmaps a file(R--), and uses write syscall to change > trampolines in it. > No matter how you do it, these are all user-level methods that can be hacked. The kernel cannot be sure that an attacker's code has not found its way into the file. >> 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? I don't know about the latter part. I guess I need to think about it. But the former is valid. When a page is RW-, a hacker could hack the page. Then it does not matter that the page is transitioned to R--. Again, the kernel cannot be sure that the user has verified the contents after R--. IMO, W^X needs to be enforced temporally as well. Madhavan