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=unavailable 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 DD5CCC2D0EF for ; Fri, 17 Apr 2020 15:38:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C33C921924 for ; Fri, 17 Apr 2020 15:38:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729200AbgDQPiB (ORCPT ); Fri, 17 Apr 2020 11:38:01 -0400 Received: from mout-p-101.mailbox.org ([80.241.56.151]:56610 "EHLO mout-p-101.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729008AbgDQPiA (ORCPT ); Fri, 17 Apr 2020 11:38:00 -0400 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-101.mailbox.org (Postfix) with ESMTPS id 493gGF63gjzKmbF; Fri, 17 Apr 2020 17:37:57 +0200 (CEST) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp1.mailbox.org ([80.241.60.240]) by spamfilter06.heinlein-hosting.de (spamfilter06.heinlein-hosting.de [80.241.56.125]) (amavisd-new, port 10030) with ESMTP id Le9MmvzJwhqy; Fri, 17 Apr 2020 17:37:51 +0200 (CEST) Date: Sat, 18 Apr 2020 01:37:40 +1000 From: Aleksa Sarai To: "Michael Kerrisk (man-pages)" Cc: Al Viro , Christian Brauner , Aleksa Sarai , linux-man , Linux API , lkml Subject: Re: [PATCH RFC 1/3] symlink.7: document magic-links more completely Message-ID: <20200417153740.37j2uxjcasyieuoa@yavin.dot.cyphar.com> References: <20191003145542.17490-1-cyphar@cyphar.com> <20191003145542.17490-2-cyphar@cyphar.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="syb7vzwyvrr2nafi" Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: A7CA31805 X-Rspamd-Score: -6.73 / 15.00 / 15.00 Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org --syb7vzwyvrr2nafi Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2020-04-17, Michael Kerrisk (man-pages) wrote: > Hi Aleksa, >=20 > Re our discussion of documentation to be added for magic symlinks, > there was the patch below, which got paused. I guess this just needs a > light refresh? Yes, this is the patch I was thinking of -- but since the whole "magic link mode" semantics weren't in the openat2() series that was merged, this would need a refresh. Also I feel that magic-links probably deserve a slightly longer explanation than I gave here. > Thanks, >=20 > Michael >=20 > On Thu, 3 Oct 2019 at 16:56, Aleksa Sarai wrote: > > > > Traditionally, magic-links have not been a well-understood topic in > > Linux. Given the new changes in their semantics (related to the link > > mode of trailing magic-links), it seems like a good opportunity to shine > > more light on magic-links and their semantics. > > > > Signed-off-by: Aleksa Sarai > > --- > > man7/path_resolution.7 | 15 +++++++++++++++ > > man7/symlink.7 | 39 ++++++++++++++++++++++++++++++--------- > > 2 files changed, 45 insertions(+), 9 deletions(-) > > > > diff --git a/man7/path_resolution.7 b/man7/path_resolution.7 > > index 07664ed8faec..46f25ec4cdfa 100644 > > --- a/man7/path_resolution.7 > > +++ b/man7/path_resolution.7 > > @@ -136,6 +136,21 @@ we are just creating it. > > The details on the treatment > > of the final entry are described in the manual pages of the specific > > system calls. > > +.PP > > +Since Linux 5.FOO, if the final entry is a "magic-link" (see > > +.BR symlink (7)), > > +and the user is attempting to > > +.BR open (2) > > +it, then there is an additional permission-related restriction applied= to the > > +operation: the requested access mode must not exceed the "link mode" o= f the > > +magic-link (unlike ordinary symlinks, magic-links have their own file = mode.) > > +For example, if > > +.I /proc/[pid]/fd/[num] > > +has a link mode of > > +.BR 0500 , > > +unprivileged users are not permitted to > > +.BR open () > > +the magic-link for writing. > > .SS . and .. > > By convention, every directory has the entries "." and "..", > > which refer to the directory itself and to its parent directory, > > diff --git a/man7/symlink.7 b/man7/symlink.7 > > index 9f5bddd5dc21..33f0ec703acd 100644 > > --- a/man7/symlink.7 > > +++ b/man7/symlink.7 > > @@ -84,6 +84,25 @@ as they are implemented on Linux and other systems, > > are outlined here. > > It is important that site-local applications also conform to these rul= es, > > so that the user interface can be as consistent as possible. > > +.SS Magic-links > > +There is a special class of symlink-like objects known as "magic-links= " which > > +can be found in certain pseudo-filesystems such as > > +.BR proc (5) > > +(examples include > > +.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .) > > +Unlike normal symlinks, magic-links are not resolved through > > +pathname-expansion, but instead act as direct references to the kernel= 's own > > +representation of a file handle. As such, these magic-links allow user= s to > > +access files which cannot be referenced with normal paths (such as unl= inked > > +files still referenced by a running program.) > > +.PP > > +Because they can bypass ordinary > > +.BR mount_namespaces (7)-based > > +restrictions, magic-links have been used as attack vectors in various = exploits. > > +As such (since Linux 5.FOO), there are additional restrictions placed = on the > > +re-opening of magic-links (see > > +.BR path_resolution (7) > > +for more details.) > > .SS Symbolic link ownership, permissions, and timestamps > > The owner and group of an existing symbolic link can be changed > > using > > @@ -99,16 +118,18 @@ of a symbolic link can be changed using > > or > > .BR lutimes (3). > > .PP > > -On Linux, the permissions of a symbolic link are not used > > -in any operations; the permissions are always > > -0777 (read, write, and execute for all user categories), > > .\" Linux does not currently implement an lchmod(2). > > -and can't be changed. > > -(Note that there are some "magic" symbolic links in the > > -.I /proc > > -directory tree\(emfor example, the > > -.IR /proc/[pid]/fd/* > > -files\(emthat have different permissions.) > > +On Linux, the permissions of an ordinary symbolic link are not used in= any > > +operations; the permissions are always 0777 (read, write, and execute = for all > > +user categories), and can't be changed. > > +.PP > > +However, magic-links do not follow this rule. They can have a non-0777= mode, > > +which is used for permission checks when the final > > +component of an > > +.BR open (2)'s > > +path is a magic-link (see > > +.BR path_resolution (7).) > > + > > .\" > > .\" The > > .\" 4.4BSD > > -- > > 2.23.0 > > >=20 >=20 > --=20 > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Linux/UNIX System Programming Training: http://man7.org/training/ --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --syb7vzwyvrr2nafi Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXpnNQAAKCRCdlLljIbnQ EtJxAP4iVwtZsIQJsWRPvKFpLrO+MuKBOHZ8y5pMdV7znPB54gEAkujcyOtXHdtp 2VLvRGYOKn939L3xgsrEqZSUCRhrSg4= =YtrC -----END PGP SIGNATURE----- --syb7vzwyvrr2nafi--