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=-4.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 48771C433E0 for ; Tue, 11 Aug 2020 14:03:17 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 4A3F320716 for ; Tue, 11 Aug 2020 14:03:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="eVOXFZGT" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4A3F320716 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19595-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 13604 invoked by uid 550); 11 Aug 2020 14:03:09 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 13578 invoked from network); 11 Aug 2020 14:03:08 -0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=5rQ5dY74xtq7tFdjS2KoPSsnFzw7cy9K3d3kbAwfqoI=; b=eVOXFZGTb4S65kl2EyHtA0JBK5 bgr8ijcvRiK7aiP7g5WnQk0cTWdwGgzw9q4XrrFEl8DqvjlqAjPUsmEepQhRq/y68VWi+QDIUtP21 +IZC/aY3ijRtUDmWEo+EWCS/N4/wJBwsODAR9Y1SMHbB3OUAt8eG3S9AL2S7y/5x4rMGCL68ZVPEW NRaHToy51WdE1+l6tz/iPd4zzzRJ2bblwtT4nXdXTDiAStxtwIFAb8062CFjQjb6voRgo0BADPtaD c8l9fhcKj/rtg3Gql938sVm06wFUm+0Na5ugLcYYvERnDeSlwgWgLQz9hhXDUy3cBZQddwYil8Nb4 k3FJa4dQ==; Date: Tue, 11 Aug 2020 15:02:03 +0100 From: Matthew Wilcox To: Mimi Zohar Cc: =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= , Jann Horn , Kees Cook , Deven Bowers , Al Viro , Andrew Morton , kernel list , Aleksa Sarai , Alexei Starovoitov , Andy Lutomirski , Christian Brauner , Christian Heimes , Daniel Borkmann , Dmitry Vyukov , Eric Biggers , Eric Chiang , Florian Weimer , James Morris , Jan Kara , Jonathan Corbet , Lakshmi Ramasubramanian , Matthew Garrett , Michael Kerrisk , Philippe =?iso-8859-1?Q?Tr=E9buchet?= , Scott Shell , Sean Christopherson , Shuah Khan , Steve Dower , Steve Grubb , Tetsuo Handa , Thibaut Sautereau , Vincent Strubel , Kernel Hardening , Linux API , linux-integrity@vger.kernel.org, linux-security-module , linux-fsdevel Subject: Re: [PATCH v7 0/7] Add support for O_MAYEXEC Message-ID: <20200811140203.GQ17456@casper.infradead.org> References: <20200723171227.446711-1-mic@digikod.net> <202007241205.751EBE7@keescook> <0733fbed-cc73-027b-13c7-c368c2d67fb3@digikod.net> <20200810202123.GC1236603@ZenIV.linux.org.uk> <917bb071-8b1a-3ba4-dc16-f8d7b4cc849f@digikod.net> <0cc94c91-afd3-27cd-b831-8ea16ca8ca93@digikod.net> <5db0ef9cb5e7e1569a5a1f7a0594937023f7290b.camel@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5db0ef9cb5e7e1569a5a1f7a0594937023f7290b.camel@linux.ibm.com> On Tue, Aug 11, 2020 at 09:56:50AM -0400, Mimi Zohar wrote: > On Tue, 2020-08-11 at 10:48 +0200, Mickaël Salaün wrote: > > On 11/08/2020 01:03, Jann Horn wrote: > > > On Tue, Aug 11, 2020 at 12:43 AM Mickaël Salaün wrote: > > > > On 10/08/2020 22:21, Al Viro wrote: > > > > > On Mon, Aug 10, 2020 at 10:11:53PM +0200, Mickaël Salaün wrote: > > > > > > It seems that there is no more complains nor questions. Do you want me > > > > > > to send another series to fix the order of the S-o-b in patch 7? > > > > > > > > > > There is a major question regarding the API design and the choice of > > > > > hooking that stuff on open(). And I have not heard anything resembling > > > > > a coherent answer. > > > > > > > > Hooking on open is a simple design that enables processes to check files > > > > they intend to open, before they open them. From an API point of view, > > > > this series extends openat2(2) with one simple flag: O_MAYEXEC. The > > > > enforcement is then subject to the system policy (e.g. mount points, > > > > file access rights, IMA, etc.). > > > > > > > > Checking on open enables to not open a file if it does not meet some > > > > requirements, the same way as if the path doesn't exist or (for whatever > > > > reasons, including execution permission) if access is denied. > > > > > > You can do exactly the same thing if you do the check in a separate > > > syscall though. > > > > > > And it provides a greater degree of flexibility; for example, you can > > > use it in combination with fopen() without having to modify the > > > internals of fopen() or having to use fdopen(). > > > > > > > It is a > > > > good practice to check as soon as possible such properties, and it may > > > > enables to avoid (user space) time-of-check to time-of-use (TOCTOU) > > > > attacks (i.e. misuse of already open resources). > > > > > > The assumption that security checks should happen as early as possible > > > can actually cause security problems. For example, because seccomp was > > > designed to do its checks as early as possible, including before > > > ptrace, we had an issue for a long time where the ptrace API could be > > > abused to bypass seccomp filters. > > > > > > Please don't decide that a check must be ordered first _just_ because > > > it is a security check. While that can be good for limiting attack > > > surface, it can also create issues when the idea is applied too > > > broadly. > > > > I'd be interested with such security issue examples. > > > > I hope that delaying checks will not be an issue for mechanisms such as > > IMA or IPE: > > https://lore.kernel.org/lkml/1544699060.6703.11.camel@linux.ibm.com/ > > > > Any though Mimi, Deven, Chrome OS folks? > > One of the major gaps, defining a system wide policy requiring all code > being executed to be signed, is interpreters. The kernel has no > context for the interpreter's opening the file. From an IMA > perspective, this information needs to be conveyed to the kernel prior > to ima_file_check(), which would allow IMA policy rules to be defined > in terms of O_MAYEXEC. This is kind of evading the point -- Mickaël is proposing a new flag to open() to tell IMA to measure the file being opened before the fd is returned to userspace, and Al is suggesting a new syscall to allow a previously-obtained fd to be measured. I think what you're saying is that you don't see any reason to prefer one over the other.