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=-0.7 required=3.0 tests=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 732F3C43215 for ; Thu, 14 Nov 2019 13:33:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4DE47206E6 for ; Thu, 14 Nov 2019 13:33:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726528AbfKNNdj (ORCPT ); Thu, 14 Nov 2019 08:33:39 -0500 Received: from mout-p-201.mailbox.org ([80.241.56.171]:21232 "EHLO mout-p-201.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726202AbfKNNdj (ORCPT ); Thu, 14 Nov 2019 08:33:39 -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-201.mailbox.org (Postfix) with ESMTPS id 47DMrH5rJDzQl9x; Thu, 14 Nov 2019 14:33:35 +0100 (CET) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp1.mailbox.org ([80.241.60.240]) by spamfilter03.heinlein-hosting.de (spamfilter03.heinlein-hosting.de [80.241.56.117]) (amavisd-new, port 10030) with ESMTP id Wtrcg9lX2PUE; Thu, 14 Nov 2019 14:33:30 +0100 (CET) Date: Fri, 15 Nov 2019 00:33:00 +1100 From: Aleksa Sarai To: Al Viro Cc: Jeff Layton , "J. Bruce Fields" , Arnd Bergmann , David Howells , Shuah Khan , Shuah Khan , Ingo Molnar , Peter Zijlstra , Christian Brauner , David Drysdale , Andy Lutomirski , Linus Torvalds , Eric Biederman , Andrew Morton , Alexei Starovoitov , Kees Cook , Jann Horn , Tycho Andersen , Chanho Min , Oleg Nesterov , Rasmus Villemoes , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Christian Brauner , Aleksa Sarai , containers@lists.linux-foundation.org, linux-alpha@vger.kernel.org, linux-api@vger.kernel.org, libc-alpha@sourceware.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org Subject: Re: [PATCH v15 3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing Message-ID: <20191114133300.soxnzmufwbt2ddid@yavin.dot.cyphar.com> References: <20191105090553.6350-1-cyphar@cyphar.com> <20191105090553.6350-4-cyphar@cyphar.com> <20191113013630.GZ26530@ZenIV.linux.org.uk> <20191114044945.ldedzjrb4s7i7irr@yavin.dot.cyphar.com> <20191114054348.GH26530@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="zoh3k636biknnjmo" Content-Disposition: inline In-Reply-To: <20191114054348.GH26530@ZenIV.linux.org.uk> Sender: linux-m68k-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-m68k@vger.kernel.org Message-ID: <20191114133300.6wbSt74OpHFi8iXtrJTyX92NLvv4-aOZaqZsz0dqoaQ@z> --zoh3k636biknnjmo Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-11-14, Al Viro wrote: > On Thu, Nov 14, 2019 at 03:49:45PM +1100, Aleksa Sarai wrote: > > On 2019-11-13, Al Viro wrote: > > > On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote: > > >=20 > > > > @@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd) > > > > void nd_jump_link(struct path *path) > > > > { > > > > struct nameidata *nd =3D current->nameidata; > > > > + > > > > + nd->last_magiclink.same_mnt =3D (nd->path.mnt =3D=3D path->mnt); > > > > path_put(&nd->path); > > > > =20 > > > > nd->path =3D *path; > > > > @@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd) > > > > if (nd->flags & LOOKUP_MAGICLINK_JUMPED) { > > > > if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) > > > > return ERR_PTR(-ELOOP); > > > > + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { > > > > + if (!nd->last_magiclink.same_mnt) > > > > + return ERR_PTR(-EXDEV); > > > > + } > > > > } > > >=20 > > > Ugh... Wouldn't it be better to take that logics (some equivalent th= ereof) > > > into nd_jump_link()? Or just have nd_jump_link() return an error... > >=20 > > This could be done, but the reason for stashing it away in > > last_magiclink is because of the future magic-link re-opening patches > > which can't be implemented like that without putting the open_flags > > inside nameidata (which was decided to be too ugly a while ago). > >=20 > > My point being that I could implement it this way for this series, but > > I'd have to implement something like last_magiclink when I end up > > re-posting the magic-link stuff in a few weeks. > >=20 > > Looking at all the nd_jump_link() users, the other option is to just > > disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only > > thing allowing them permits is to resolve file descriptors that are > > pointing to the same procfs mount -- and it's unclear to me how useful > > that really is (apparmorfs and nsfs will always give -EXDEV because > > aafs_mnt and nsfs_mnt are internal kernel vfsmounts). >=20 > I would rather keep the entire if (nd->flags & LOOKUP_MAGICLINK_JUMPED) > out of the get_link(). If you want to generate some error if > nd_jump_link() has been called, just do it right there. The fewer > pieces of state need to be carried around, the better... Sure, I can make nd_jump_link() give -ELOOP and drop the current need for LOOKUP_MAGICLINK_JUMPED -- if necessary we can re-add it for the magic-link reopening patches. > And as for opening them... Why would you need full open_flags in there? > Details, please... I was referring to [1] which has been dropped from this series. I misspoke -- you don't need the full open_flags, you just need acc_mode in nameidata -- but from memory you (understandably) weren't in favour of that either because it further muddled the open semantics with namei. So the solution I went with was to stash away the i_mode of the magiclink in nd->last_magiclink.mode (though to avoid a race which Jann found, you actually need to recalculate it when you call nd_jump_link() but that's a different topic) and then check it in trailing_magiclink(). However, I've since figured out that we need to restrict things like bind-mounts and truncate() because they can be used to get around the restrictions. I dropped that patch from this series so that I could work on implementing the restrictions for the other relevant VFS syscalls separately from openat2 (upgrade_mask will be re-added to open_how with those patches). My point was that AFAICS we will either have to have nd->acc_mode (or something similar) or have nd->last_magiclink in order to implement the magic-link reopening hardening. [1]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@cyphar.com/ --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --zoh3k636biknnjmo Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXc1XiQAKCRCdlLljIbnQ EgEVAQDde9bpKjJAbLEIt4D/9cw3B8CHqEBeW8SnIT4PqQNQUQD/TtE4FY5p3N1d gPZFRde/N3ihwtWscDvPXctFNxykJAQ= =TSKH -----END PGP SIGNATURE----- --zoh3k636biknnjmo--