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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 A538BC43331 for ; Fri, 6 Sep 2019 17:21:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 806592067B for ; Fri, 6 Sep 2019 17:21:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732785AbfIFRVA (ORCPT ); Fri, 6 Sep 2019 13:21:00 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:54469 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729928AbfIFRVA (ORCPT ); Fri, 6 Sep 2019 13:21:00 -0400 Received: from [213.220.153.21] (helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.76) (envelope-from ) id 1i6Hum-000463-IN; Fri, 06 Sep 2019 17:20:52 +0000 Date: Fri, 6 Sep 2019 19:20:51 +0200 From: Christian Brauner To: Aleksa Sarai Cc: =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= , Florian Weimer , =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= , linux-kernel@vger.kernel.org, Alexei Starovoitov , Al Viro , Andy Lutomirski , Christian Heimes , Daniel Borkmann , Eric Chiang , James Morris , Jan Kara , Jann Horn , Jonathan Corbet , Kees Cook , Matthew Garrett , Matthew Wilcox , Michael Kerrisk , Mimi Zohar , Philippe =?utf-8?Q?Tr=C3=A9buchet?= , Scott Shell , Sean Christopherson , Shuah Khan , Song Liu , Steve Dower , Steve Grubb , Thibaut Sautereau , Vincent Strubel , Yves-Alexis Perez , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() Message-ID: <20190906172050.v44f43psd6qc6awi@wittgenstein> References: <20190906152455.22757-1-mic@digikod.net> <20190906152455.22757-2-mic@digikod.net> <87ef0te7v3.fsf@oldenburg2.str.redhat.com> <75442f3b-a3d8-12db-579a-2c5983426b4d@ssi.gouv.fr> <20190906170739.kk3opr2phidb7ilb@yavin.dot.cyphar.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190906170739.kk3opr2phidb7ilb@yavin.dot.cyphar.com> User-Agent: NeoMutt/20180716 Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: On Sat, Sep 07, 2019 at 03:07:39AM +1000, Aleksa Sarai wrote: > On 2019-09-06, Mickaël Salaün wrote: > > > > On 06/09/2019 17:56, Florian Weimer wrote: > > > Let's assume I want to add support for this to the glibc dynamic loader, > > > while still being able to run on older kernels. > > > > > > Is it safe to try the open call first, with O_MAYEXEC, and if that fails > > > with EINVAL, try again without O_MAYEXEC? > > > > The kernel ignore unknown open(2) flags, so yes, it is safe even for > > older kernel to use O_MAYEXEC. > > Depends on your definition of "safe" -- a security feature that you will > silently not enable on older kernels doesn't sound super safe to me. > Unfortunately this is a limitation of open(2) that we cannot change -- > which is why the openat2(2) proposal I've been posting gives -EINVAL for > unknown O_* flags. > > There is a way to probe for support (though unpleasant), by creating a > test O_MAYEXEC fd and then checking if the flag is present in > /proc/self/fdinfo/$n. Which Florian said they can't do for various reasons. It is a major painpoint if there's no easy way for userspace to probe for support. Especially if it's security related which usually means that you want to know whether this feature works or not. Christian