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Christian Brauner 52c081b02c
new{g,u}idmap: align setuid and fscaps behavior 
Commit 1ecca8439d ("new[ug]idmap: not require CAP_SYS_ADMIN in the parent userNS")
does contain a wrong commit message, is lacking an explanation of the
issue, misses some simplifications and hardening features. This commit
tries to rectify this.

In (crazy) environment where all capabilities are dropped from the
capability bounding set apart from CAP_SET{G,U}ID setuid- and
fscaps-based new{g,u}idmap binaries behave differently when writing
complex mappings for an unprivileged user:

1. newuidmap is setuid

unshare -U sleep infinity &
newuidmap $? 0 100000 65536

First file_ns_capable(file, ns, CAP_SYS_ADMIN) is hit. This calls into
cap_capable() and hits the loop

for (;;) {
        /* Do we have the necessary capabilities? */
        if (ns == cred->user_ns)
                return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;

        /*
         * If we're already at a lower level than we're looking for,
         * we're done searching.
         */
        if (ns->level <= cred->user_ns->level)
                return -EPERM;

        /*
         * The owner of the user namespace in the parent of the
         * user namespace has all caps.
        */
        if ((ns->parent == cred->user_ns) && uid_eq(ns->owner, cred->euid))
                return 0;

        /*
         * If you have a capability in a parent user ns, then you have
         * it over all children user namespaces as well.
        */
        ns = ns->parent;
}

The first check fails and falls through to the end of the loop and
retrieves the parent user namespace and checks whether CAP_SYS_ADMIN is
available there which isn't.

2. newuidmap has CAP_SETUID as fscaps set

unshare -U sleep infinity &
newuidmap $? 0 100000 65536

The first file_ns_capable() check for CAP_SYS_ADMIN is passed since the
euid has not been changed:

if ((ns->parent == cred->user_ns) && uid_eq(ns->owner, cred->euid))
        return 0;

Now new_idmap_permitted() is hit which calls ns_capable(ns->parent,
CAP_SET{G,U}ID). This check passes since CAP_SET{G,U}ID is available in
the parent user namespace.
Now file_ns_capable(file, ns->parent, CAP_SETUID) is hit and the
cap_capable() loop (see above) is entered again. This passes

if (ns == cred->user_ns)
        return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;

since CAP_SET{G,U}ID is available in the parent user namespace. Now the
mapping can be written.

There is no need for this descrepancy between setuid and fscaps based
new{g,u}idmap binaries. The solution is to do a
seteuid() back to the unprivileged uid and PR_SET_KEEPCAPS to keep
CAP_SET{G,U}ID. The seteuid() will cause the
file_ns_capable(file, ns, CAP_SYS_ADMIN) check to pass and the
PR_SET_KEEPCAPS for CAP_SET{G,U}ID will cause the CAP_SET{G,U}ID to
pass.

Fixes: 1ecca8439d ("new[ug]idmap: not require CAP_SYS_ADMIN in the parent userNS")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
2018-10-28 01:27:48 +02:00
contrib spelling: thanks 2017-10-22 21:24:49 +00:00
doc spelling: moment 2017-10-22 20:39:14 +00:00
etc Make language less binary 2017-09-20 17:00:29 +01:00
lib Merge pull request #118 from AdelieLinux/utmpx-only-support 2018-10-23 22:35:19 -05:00
libmisc new{g,u}idmap: align setuid and fscaps behavior 2018-10-28 01:27:48 +02:00
man Merge pull request #103 from HarmtH/be-predictable 2018-03-29 23:10:51 -07:00
po po/zh_CN: update 2018-06-16 18:26:28 +08:00
src Merge pull request #136 from giuseppe/fcap-newuidmap-newgidmap 2018-10-27 11:26:31 -05:00
tests spelling: within 2017-10-22 21:37:53 +00:00
.gitignore ignore config.cache & dist files 2016-12-05 15:48:10 -05:00
.travis.yml run travis 2016-12-06 01:09:17 +02:00
acinclude.m4 [svn-upgrade] Integrating new upstream version, shadow (4.0.13) 2007-10-07 11:47:01 +00:00
autogen.sh * autogen.sh: Restore original autoreconf (see 2010-08-29's change 2011-02-12 14:57:37 +00:00
ChangeLog release 4.6 2018-04-29 11:41:41 -05:00
configure.ac Merge pull request #136 from giuseppe/fcap-newuidmap-newgidmap 2018-10-27 11:26:31 -05:00
COPYING Added the subversion svn:keywords property (Id) for proper identification. 2007-11-10 23:46:11 +00:00
Makefile.am switch bz2 dist to xz 2016-12-05 15:57:34 -05:00
NEWS spelling: various 2017-10-22 21:33:42 +00:00
README README: add Aleksa Sarai to author list 2018-02-16 17:56:36 +11:00
shadow.spec.in * shadow.spec.in: Fix the source (new FTP). 2008-08-31 17:30:45 +00:00
TODO New entry for vipw. 2012-01-15 21:26:56 +00:00

README 

Shadow SITES
============

Homepage
	http://pkg-shadow.alioth.debian.org/

FTP site
	ftp://pkg-shadow.alioth.debian.org/pub/pkg-shadow

SVN repository
	anonymous read only access: svn://svn.debian.org/pkg-shadow/upstream

SVN web interface
	http://svn.debian.org/wsvn/pkg-shadow/upstream
	or
	http://svn.debian.org/viewsvn/pkg-shadow/upstream

Mailing lists
	for general discuss: pkg-shadow-devel@lists.alioth.debian.org
	commit list: pkg-shadow-commits@lists.alioth.debian.org

Mailing lists subscription
	http://lists.alioth.debian.org/mailman/listinfo/pkg-shadow-devel
	http://lists.alioth.debian.org/mailman/listinfo/pkg-shadow-commits

Mailing lists archives:
	http://lists.alioth.debian.org/pipermail/pkg-shadow-devel/
	http://lists.alioth.debian.org/pipermail/pkg-shadow-commits/

S/Key support:
	Shadow can be built with S/Key support using the S/Key package from:

	http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libskey/
	or
	http://gentoo.osuosl.org/distfiles/skey-1.1.5.tar.bz2

Authors and contributors
========================

Thanks to at least the following people for sending patches, bug
reports and various comments.  This list may be incomplete, I received
a lot of mail...


Adam Rudnicki <adam@v-lo.krakow.pl>
Alan Curry <pacman@tardis.mars.net>
Aleksa Sarai <cyphar@cyphar.com>
Alexander O. Yuriev <alex@bach.cis.temple.edu>
Algis Rudys <arudys@rice.edu>
Andreas Jaeger <aj@arthur.rhein-neckar.de>
Aniello Del Sorbo <anidel@edu-gw.dia.unisa.it>
Anton Gluck <gluc@midway.uchicago.edu>
Arkadiusz Miskiewicz <misiek@pld.org.pl>
Ben Collins <bcollins@debian.org>
Brian R. Gaeke <brg@dgate.org>
Calle Karlsson <ckn@kash.se>
Chip Rosenthal <chip@unicom.com>
Chris Evans <lady0110@sable.ox.ac.uk>
Cristian Gafton <gafton@sorosis.ro>
Dan Walsh <dwalsh@redhat.com>
Darcy Boese <possum@chardonnay.niagara.com>
Dave Hagewood <admin@arrowweb.com>
David A. Holland <dholland@hcs.harvard.edu>
David Frey <David.Frey@lugs.ch>
Ed Carp <ecarp@netcom.com>
Floody <flood@evcom.net>
Frank Denis <j@4u.net>
George Kraft IV <gk4@us.ibm.com>
Greg Mortensen <loki@world.std.com>
Guido van Rooij
Guy Maor <maor@debian.org>
Hrvoje Dogan <hdogan@bjesomar.srce.hr>
Jakub Hrozek <jhrozek@redhat.com>
Janos Farkas <chexum@bankinf.banki.hu>
Jay Soffian <jay@lw.net>
Jesse Thilo <Jesse.Thilo@pobox.com>
Joey Hess <joey@kite.ml.org>
John Adelsberger <jja@umr.edu>
Jonathan Hankins <jhankins@mailserv.homewood.k12.al.us>
Jon Lewis <jlewis@lewis.org>
Joshua Cowan <jcowan@hermit.reslife.okstate.edu>
Judd Bourgeois <shagboy@bluesky.net>
Juergen Heinzl <unicorn@noris.net>
Juha Virtanen <jiivee@iki.fi>
Julian Pidancet <julian.pidancet@gmail.com>
Julianne Frances Haugh <jockgrrl@ix.netcom.com>
Leonard N. Zubkoff <lnz@dandelion.com>
Luca Berra <bluca@www.polimi.it>
Lukáš Kuklínek <lkukline@redhat.com>
Lutz Schwalowsky <schwalow@mineralogie.uni-hamburg.de>
Marc Ewing <marc@redhat.com>
Martin Bene <mb@sime.com>
Martin Mares <mj@gts.cz>
Michael Meskes <meskes@topsystem.de>
Michael Talbot-Wilson <mike@calypso.bns.com.au>
Mike Frysinger <vapier@gentoo.org>
Mike Pakovic <mpakovic@users.southeast.net>
Nicolas François <nicolas.francois@centraliens.net>
Nikos Mavroyanopoulos <nmav@i-net.paiko.gr>
Pavel Machek <pavel@bug.ucw.cz>
Peter Vrabec <pvrabec@redhat.com>
Phillip Street
Rafał Maszkowski <rzm@icm.edu.pl>
Rani Chouha <ranibey@smartec.com>
Sami Kerola <kerolasa@rocketmail.com>
Scott Garman <scott.a.garman@intel.com>
Sebastian Rick Rijkers <srrijkers@gmail.com>
Seraphim Mellos <mellos@ceid.upatras.gr>
Shane Watts <shane@nexus.mlckew.edu.au>
Steve M. Robbins <steve@nyongwa.montreal.qc.ca>
Thorsten Kukuk <kukuk@suse.de>
Tim Hockin <thockin@eagle.ais.net>
Timo Karjalainen <timok@iki.fi>
Ulisses Alonso Camaro <ulisses@pusa.eleinf.uv.es>
Werner Fink <werner@suse.de>

Maintainers
===========

Tomasz Kłoczko <kloczek@pld.org.pl> (2000-2007)
Nicolas François <nicolas.francois@centraliens.net> (2007-2014)
Serge E. Hallyn <serge@hallyn.com> (2014-now)