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[209.132.180.67]) by mx.google.com with ESMTP id h35si9176364pgm.183.2019.09.07.22.37.33; Sat, 07 Sep 2019 22:37:48 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404292AbfIFWS2 (ORCPT + 99 others); Fri, 6 Sep 2019 18:18:28 -0400 Received: from mx2.mailbox.org ([80.241.60.215]:13044 "EHLO mx2.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731008AbfIFWS1 (ORCPT ); Fri, 6 Sep 2019 18:18:27 -0400 Received: from smtp2.mailbox.org (smtp2.mailbox.org [IPv6:2001:67c:2050:105:465:1:2:0]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mx2.mailbox.org (Postfix) with ESMTPS id 213A3A1068; Sat, 7 Sep 2019 00:18:23 +0200 (CEST) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp2.mailbox.org ([80.241.60.241]) by gerste.heinlein-support.de (gerste.heinlein-support.de [91.198.250.173]) (amavisd-new, port 10030) with ESMTP id htMwsCvpaLvv; Sat, 7 Sep 2019 00:18:19 +0200 (CEST) Date: Sat, 7 Sep 2019 08:18:00 +1000 From: Aleksa Sarai To: Jeff Layton 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: <20190906221800.xzowcjgdeljikcyu@yavin.dot.cyphar.com> 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> <20190906171335.d7mc3no5tdrcn6r5@yavin.dot.cyphar.com> <20190906220546.gkqxzm4y3ynevvtz@yavin.dot.cyphar.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="punotohnhmzxr25f" Content-Disposition: inline In-Reply-To: <20190906220546.gkqxzm4y3ynevvtz@yavin.dot.cyphar.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --punotohnhmzxr25f Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-09-07, Aleksa Sarai wrote: > On 2019-09-06, Jeff Layton wrote: > > On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote: > > > On 2019-09-06, Jeff Layton wrote: > > > > On Fri, 2019-09-06 at 18:06 +0200, Micka=EBl Sala=FCn wrote: > > > > > On 06/09/2019 17:56, Florian Weimer wrote: > > > > > > Let's assume I want to add support for this to the glibc dynami= c loader, > > > > > > while still being able to run on older kernels. > > > > > >=20 > > > > > > Is it safe to try the open call first, with O_MAYEXEC, and if t= hat fails > > > > > > with EINVAL, try again without O_MAYEXEC? > > > > >=20 > > > > > The kernel ignore unknown open(2) flags, so yes, it is safe even = for > > > > > older kernel to use O_MAYEXEC. > > > > >=20 > > > >=20 > > > > Well...maybe. What about existing programs that are sending down bo= gus > > > > open flags? Once you turn this on, they may break...or provide a wa= y to > > > > circumvent the protections this gives. > > >=20 > > > It should be noted that this has been a valid concern for every new O= _* > > > flag introduced (and yet we still introduced new flags, despite the > > > concern) -- though to be fair, O_TMPFILE actually does have a > > > work-around with the O_DIRECTORY mask setup. > > >=20 > > > The openat2() set adds O_EMPTYPATH -- though in fairness it's also > > > backwards compatible because empty path strings have always given ENO= ENT > > > (or EINVAL?) while O_EMPTYPATH is a no-op non-empty strings. > > >=20 > > > > Maybe this should be a new flag that is only usable in the new open= at2() > > > > syscall that's still under discussion? That syscall will enforce th= at > > > > all flags are recognized. You presumably wouldn't need the sysctl i= f you > > > > went that route too. > > >=20 > > > I'm also interested in whether we could add an UPGRADE_NOEXEC flag to > > > how->upgrade_mask for the openat2(2) patchset (I reserved a flag bit = for > > > it, since I'd heard about this work through the grape-vine). > > >=20 > >=20 > > I rather like the idea of having openat2 fds be non-executable by > > default, and having userland request it specifically via O_MAYEXEC (or > > some similar openat2 flag) if it's needed. Then you could add an > > UPGRADE_EXEC flag instead? > >=20 > > That seems like something reasonable to do with a brand new API, and > > might be very helpful for preventing certain classes of attacks. >=20 > In that case, maybe openat2(2) should default to not allowing any > upgrades by default? The reason I pitched UPGRADE_NOEXEC is because > UPGRADE_NO{READ,WRITE} are the existing @how->upgrade_mask flags. Sorry, another issue is that there isn't a current way to really restrict fexecve() permissions (from my [limited] understanding, __FMODE_EXEC isn't the right thing to use) -- so we can't blanket block exec through openat2() O_PATH descriptors and add UPGRADE_EXEC later. We would have to implement FMODE_EXEC (and FMODE_MAP_EXEC as you suggested) in order to implement FMODE_UPGRADE_EXEC before we could even get a first version of openat2(2) in. Though, I do (a little begrudgingly) agree that we should have a safe default if possible (magical O_PATH reopening trickery is something that most people don't know about and probably wouldn't want to happen if they did). --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --punotohnhmzxr25f Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXXLbFQAKCRCdlLljIbnQ Ep08APwP1U/HfIWYfQu5c32xbumkvXiBXlVt7ZbRhD2w2qfdiQEA4cduDnL5ROTb EPmEhurBIVJ5eX/EZj59be1o07F3JgY= =wxD8 -----END PGP SIGNATURE----- --punotohnhmzxr25f--