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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 BF3E7C2D0C0 for ; Fri, 27 Dec 2019 02:31:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9870A20828 for ; Fri, 27 Dec 2019 02:31:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727132AbfL0Cbr (ORCPT ); Thu, 26 Dec 2019 21:31:47 -0500 Received: from mout-p-102.mailbox.org ([80.241.56.152]:20124 "EHLO mout-p-102.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726277AbfL0Cbq (ORCPT ); Thu, 26 Dec 2019 21:31:46 -0500 Received: from smtp1.mailbox.org (smtp1.mailbox.org [IPv6:2001:67c:2050:105:465:1:1:0]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mout-p-102.mailbox.org (Postfix) with ESMTPS id 47kW6l6W6pzKmXM; Fri, 27 Dec 2019 03:31:43 +0100 (CET) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp1.mailbox.org ([80.241.60.240]) by spamfilter05.heinlein-hosting.de (spamfilter05.heinlein-hosting.de [80.241.56.123]) (amavisd-new, port 10030) with ESMTP id jgUeRFQtEi48; Fri, 27 Dec 2019 03:31:40 +0100 (CET) Date: Fri, 27 Dec 2019 13:31:31 +1100 From: Aleksa Sarai To: Christian Brauner Cc: Sargun Dhillon , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, tycho@tycho.ws, jannh@google.com, keescook@chromium.org Subject: Re: [PATCH] seccomp: Check flags on seccomp_notif is unset Message-ID: <20191227023131.klnobtlfgeqcmvbb@yavin.dot.cyphar.com> References: <20191225214530.GA27780@ircssh-2.c.rugged-nimbus-611.internal> <20191226115245.usf7z5dkui7ndp4w@wittgenstein> <20191226143229.sbopynwut2hhsiwn@yavin.dot.cyphar.com> <57C06925-0CC6-4251-AD57-8FF1BC28F049@ubuntu.com> <20191227022446.37e64ag4uaqms2w4@yavin.dot.cyphar.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="hmk3t7hh5batxsb5" Content-Disposition: inline In-Reply-To: <20191227022446.37e64ag4uaqms2w4@yavin.dot.cyphar.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --hmk3t7hh5batxsb5 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-12-27, Aleksa Sarai wrote: > On 2019-12-26, Christian Brauner wrote: > > On December 26, 2019 3:32:29 PM GMT+01:00, Aleksa Sarai wrote: > > >On 2019-12-26, Christian Brauner wrote: > > >> On Wed, Dec 25, 2019 at 09:45:33PM +0000, Sargun Dhillon wrote: > > >> > This patch is a small change in enforcement of the uapi for > > >> > SECCOMP_IOCTL_NOTIF_RECV ioctl. Specificaly, the datastructure > > >which is > > >> > passed (seccomp_notif), has a flags member. Previously that could > > >be > > >> > set to a nonsense value, and we would ignore it. This ensures that > > >> > no flags are set. > > >> >=20 > > >> > Signed-off-by: Sargun Dhillon > > >> > Cc: Kees Cook > > >>=20 > > >> I'm fine with this since we soon want to make use of the flag > > >argument > > >> when we add a flag to get a pidfd from the seccomp notifier on > > >receive. > > >> The major users I could identify already pass in seccomp_notif with > > >all > > >> fields set to 0. If we really break users we can always revert; this > > >> seems very unlikely to me though. > > >>=20 > > >> One more question below, otherwise: > > >>=20 > > >> Reviewed-by: Christian Brauner > > >>=20 > > >> > --- > > >> > kernel/seccomp.c | 7 +++++++ > > >> > 1 file changed, 7 insertions(+) > > >> >=20 > > >> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c > > >> > index 12d2227e5786..455925557490 100644 > > >> > --- a/kernel/seccomp.c > > >> > +++ b/kernel/seccomp.c > > >> > @@ -1026,6 +1026,13 @@ static long seccomp_notify_recv(struct > > >seccomp_filter *filter, > > >> > struct seccomp_notif unotif; > > >> > ssize_t ret; > > >> > =20 > > >> > + if (copy_from_user(&unotif, buf, sizeof(unotif))) > > >> > + return -EFAULT; > > >> > + > > >> > + /* flags is reserved right now, make sure it's unset */ > > >> > + if (unotif.flags) > > >> > + return -EINVAL; > > >> > + > > >>=20 > > >> Might it make sense to use > > >>=20 > > >> err =3D copy_struct_from_user(&unotif, sizeof(unotif), buf, > > >sizeof(unotif)); > > >> if (err) > > >> return err; > > >>=20 > > >> This way we check that the whole struct is 0 and report an error as > > >soon > > >> as one of the members is non-zero. That's more drastic but it'd > > >ensure > > >> that other fields can be used in the future for whatever purposes. > > >> It would also let us get rid of the memset() below.=20 > > > > > >Given that this isn't an extensible struct, it would be simpler to just > > >do > > >check_zeroed_user() -- copy_struct_from_user() is overkill. That would > > >also remove the need for any copy_from_user()s and the memset can be > > >dropped by just doing > > > > > > struct seccomp_notif unotif =3D {}; > > > > > >> > memset(&unotif, 0, sizeof(unotif)); > > >> > =20 > > >> > ret =3D down_interruptible(&filter->notif->request); > > >> > --=20 > > >> > 2.20.1 > > >> >=20 > >=20 > > It is an extensible struct. That's why we have notifier size checking b= uilt in. >=20 > Ah right, NOTIF_GET_SIZES. I reckon check_zeroed_user() is still a bit > simpler since none of the fields are used right now (and really, this > patch should be checking all of them, not just ->flags, if we want to > use any of them in the future). Scratch that -- as Tycho just mentioned, there is un-named padding in the struct so check_zeroed_user() is the wrong thing to do. But this also will make extensions harder to deal with because (presumably) they will also have un-named padding, making copy_struct_from_user() the wrong thing to do as well. So while there's not much to be done to fix the current struct layout, I humbly suggest that any future struct extensions should not have any un-named padding (so that at the very least you could use copy_struct_from_user() in some form). --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --hmk3t7hh5batxsb5 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXgVtAAAKCRCdlLljIbnQ EpCsAQD8hZGnO2be8H2eL3UnK3gHJS2WBaSQMxOmRuPONZlL4gEAgU4gEuGjnx5E /OXBhqnznUT/+Kc5/Hujvo+y2TTbIws= =MkQb -----END PGP SIGNATURE----- --hmk3t7hh5batxsb5--