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 E15A5C2D0C0 for ; Fri, 27 Dec 2019 02:28:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B96FE20828 for ; Fri, 27 Dec 2019 02:28:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727169AbfL0C2j (ORCPT ); Thu, 26 Dec 2019 21:28:39 -0500 Received: from mout-p-202.mailbox.org ([80.241.56.172]:34714 "EHLO mout-p-202.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726375AbfL0C2j (ORCPT ); Thu, 26 Dec 2019 21:28:39 -0500 Received: from smtp1.mailbox.org (smtp1.mailbox.org [80.241.60.240]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mout-p-202.mailbox.org (Postfix) with ESMTPS id 47kW381pqzzQlBc; Fri, 27 Dec 2019 03:28:36 +0100 (CET) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp1.mailbox.org ([80.241.60.240]) by spamfilter01.heinlein-hosting.de (spamfilter01.heinlein-hosting.de [80.241.56.115]) (amavisd-new, port 10030) with ESMTP id 88z-DqEwywD7; Fri, 27 Dec 2019 03:28:32 +0100 (CET) Date: Fri, 27 Dec 2019 13:28:22 +1100 From: Aleksa Sarai To: Tycho Andersen Cc: Christian Brauner , Sargun Dhillon , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, jannh@google.com, keescook@chromium.org Subject: Re: [PATCH] seccomp: Check flags on seccomp_notif is unset Message-ID: <20191227022822.v2htxtxxkrkjacem@yavin.dot.cyphar.com> References: <20191225214530.GA27780@ircssh-2.c.rugged-nimbus-611.internal> <20191226115245.usf7z5dkui7ndp4w@wittgenstein> <20191226143229.sbopynwut2hhsiwn@yavin.dot.cyphar.com> <20191226153753.GA15663@cisco> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="gcmjli2uus7kl6h5" Content-Disposition: inline In-Reply-To: <20191226153753.GA15663@cisco> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --gcmjli2uus7kl6h5 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-12-26, Tycho Andersen wrote: > On Fri, Dec 27, 2019 at 01:32:29AM +1100, 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 whic= h 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 receiv= e. > > > The major users I could identify already pass in seccomp_notif with a= ll > > > 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 secco= mp_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(u= notif)); > > > if (err) > > > return err; > > >=20 > > > This way we check that the whole struct is 0 and report an error as s= oon > > > 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 > >=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 > >=20 > > struct seccomp_notif unotif =3D {}; >=20 > This doesn't zero the padding according to the C standard, so no, you > can't drop the memset, or you may leak kernel stack bits. Ah right, I didn't double-check if there was any un-named. IMHO, It's a bit odd to have un-named padding in a struct intended for extensions (specifically to avoid these problems -- because it means userspace will pass garbage by accident and there's nothing we can do about it). But it's a bit late to worry about that now. :P --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --gcmjli2uus7kl6h5 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXgVsQgAKCRCdlLljIbnQ EnPqAQD08AhPvl6lJYODgxQHyFzeUyA/JZTLHQMKdFAhqycteAEA6IMt/FhsEspx yngR77vCilypAatAOlRiZBzPmERFdA8= =DToo -----END PGP SIGNATURE----- --gcmjli2uus7kl6h5--