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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 24B60C28CC5 for ; Sat, 8 Jun 2019 14:08:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EA61B21537 for ; Sat, 8 Jun 2019 14:08:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727107AbfFHOIn convert rfc822-to-8bit (ORCPT ); Sat, 8 Jun 2019 10:08:43 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52088 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726227AbfFHOIn (ORCPT ); Sat, 8 Jun 2019 10:08:43 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7BF86C045129; Sat, 8 Jun 2019 14:08:42 +0000 (UTC) Received: from localhost.localdomain (ovpn-116-18.phx2.redhat.com [10.3.116.18]) by smtp.corp.redhat.com (Postfix) with ESMTP id E28901001B06; Sat, 8 Jun 2019 14:08:41 +0000 (UTC) Reply-To: dwalsh@redhat.com Subject: Re: New Container vulnerability could potentially use an SELinux fix. To: Stephen Smalley , Miloslav Trmac , selinux@vger.kernel.org References: <7ca438c7-4a41-4daa-fc3f-a46a2e0af945@redhat.com> <9313f92a-46cf-c65c-6cfb-1373917de02b@tycho.nsa.gov> <75f00b22-390a-8f7b-5f84-15c64d17eb89@tycho.nsa.gov> From: Daniel Walsh Openpgp: preference=signencrypt Autocrypt: addr=dwalsh@redhat.com; prefer-encrypt=mutual; keydata= mQENBFsaqOEBCADBSnZCZpi262vX8m7iL/OdHKP9G9dhS28FR60cjd8nMPqHDNhQJBjLMZra 66L2cCIEhc4HEItail7KU1BckrMc4laFaxL8tLoVTKHZwb74n2OcAJ4FtgzkNNlB1XJvSwC/ 909uwt7cpDqwXpJvyP3t17iuklB1OY0EEjTDt9aU4+0QjHzV18L4Cpd9iQ4ksu+EHT+pjlBk DdQB+hKoAjxPl11Eh6pZfrAcrNWpYBBk0A3XE9Jb6ghbmHWltNgVOsCa9GcswJHUEeFiOup6 J5DTv6Xzwt0t6QB8nIs+wDJH+VxqAXcrxscnAhViIfGGS2AtxzjnVOz/J+UZPaauIGXTABEB AAG0LERhbmllbCBKIFdhbHNoIChGb3IgR2l0KSA8ZHdhbHNoQHJlZGhhdC5jb20+iQE4BBMB AgAiBQJbGqjhAhsDBgsJCAcDAgYVCAIJCgsEFgIDAQIeAQIXgAAKCRCi35Adq+LAKHuJB/98 nZB5RmNjMWua4Ms8q5a1R9XWlDAb3mrST6JeL+uV/M0fa18e2Aw4/hi/WZHjAjoypLmcuaRx GeCbC8iYdpfRDUG79Y956Qq+Vs8c6VfNDMY1mvtfb00eeTaYoOCu0Aa9LDeR9iLKh2g0RI+N Zr3EU45RxZdacIs1v6mU8pGpyUq/FvuTGK9GzR9d1YeVCuSpQKN4ckHNZHJUXyk0vOZft1oO nSgLqM9EDWA+yz1JLmRYwbNsim7IvfVOav5mCgnKzHcL2mLv8qCnMFZjoQV8aGny/W739Z3a YJo1CdOg6zSu5SOvmq9idYrBRkwEtyLXss2oceTVBs0MxqQ/9mLPuQENBFsaqOEBCADDl2hl bUpqJGgwt2eQvs0Z0DCx/7nn0hlLfEn4WAv2HqP25AjIRXUX31Mzu68C4QnsvNtY4zN+FGRC EfUpYsjiL7vBYlRePhIohyMYU4RLp5eXFQKahHO/9Xlhe8mwueQNwYxNBPfMQ65U2AuqxpcS scx4s5w208mhqHoKz6IB2LuKeflhYfH5Y1FNAtVGHfhg22xlcAdupPPcxGuS4fBEW6PD/SDf Y4HT5iUHsyksQKjM0IFalqZ7YuLfXBl07OD2zU7WI9c3W0dwkvwIRjt3aD4iAah544uOLff+ BzfxWghXeo80S2a1WCL0S/2qR0NVct/ExaDWboYr/bKpTa/1ABEBAAGJAR8EGAECAAkFAlsa qOECGwwACgkQot+QHaviwCi2hgf/XRvrt+VBmp1ZFxQAR9E6S7AtRT8KSytjFiqEC7TpOx3r 2OZ4gZ3ZiW4TMW8hS7aYRgF1uYpLzl7BbrCfCHfAWEcXZ+uG8vayg8G/mLAcNlLY+JE76ATs 53ziEY9R2Vb/wLMFd2nNBdqfwGcRH9N9VOej9vP76nCP01ZolY8Nms2hE383/+1Quxp5EedU BN5W5l7x9riBJyqCA63hr4u8wNsTuQgrDyhm/U1IvYeLtMopgotjnIR3KiTKOElbppLeXW3w EO/sQTPk+vQ4vcsJYY9Dnf1NlvHE4klj60GHjtjitsBEHzdE7s+J9FOxPmt8l+gMogGumKpN Y4lO0pfTyg== Organization: Red Hat Message-ID: <3957e38f-e62b-9800-da58-33ccb351a849@redhat.com> Date: Sat, 8 Jun 2019 10:08:41 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <75f00b22-390a-8f7b-5f84-15c64d17eb89@tycho.nsa.gov> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Sat, 08 Jun 2019 14:08:42 +0000 (UTC) Sender: selinux-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: selinux@vger.kernel.org On 6/7/19 5:26 PM, Stephen Smalley wrote: > On 6/7/19 5:06 PM, Daniel Walsh wrote: >> On 6/7/19 12:44 PM, Stephen Smalley wrote: >>> On 6/7/19 11:42 AM, Daniel Walsh wrote: >>>> We have periodic vulnerablities around bad container images having >>>> symbolic link attacks against the host. >>>> >>>> One came out last week about doing a `podman cp` >>>> >>>> Which would copy content from the host into the container.  The issue >>>> was that if the container was running, it could trick the processes >>>> copying content into it to follow a symbolic link to external of the >>>> container image. >>>> >>>> The question came up, is there a way to use SELinux to prevent >>>> this. And >>>> sadly the answer right now is no, because we have no way to know what >>>> the label of the process attempting to update the container file >>>> system >>>> is running as.  Usually it will be running as unconfined_t. >>>> >>>> One idea would be to add a rule to policy that control the >>>> following of >>>> symbolic links to only those specified in policy. >>>> >>>> >>>> Something like >>>> >>>> SPECIALRESTRICTED TYPE container_file_t >>>> >>>> allow container_file_t container_file_t:symlink follow; >>>> >>>> Then if a process attempted to copy content onto a symbolic link from >>>> container_file_t to a non container_file_t type, the kernel would deny >>>> access. >>>> >>>> Thoughts? >>> >>> SELinux would prevent it if you didn't allow unconfined_t (or other >>> privileged domains) to follow untrustworthy symlinks (e.g. don't allow >>> unconfined_t container_file_t:lnk_file read; in the first place). >>> That's the right way to prevent it. >>> >>> Trying to apply a check between symlink and its target as you suggest >>> is problematic; we don't generally have them both at the same point. >>> If we are allowed to follow the symlink, we read its contents and >>> perform a path walk on that, and that could be a multi-component >>> pathname lookup that itself spans further symlinks, mount points, >>> etc.  I think that would be challenging to support in the kernel, >>> subject to races, and certainly would require changes outside of just >>> SELinux. >>> >>> If you truly cannot impose such restrictions on unconfined_t, then >>> maybe podman should run in its own domain. >>> >> This is not an issue with just podman.  Podman can mount the image and >> the tools can just read/write content into the mountpoint. >> >> I thought I recalled a LSM that prefented symlink attacks when users >> would link a file in the homedir against /etc/shadow and then attempt to >> get the admin to modify the file in his homedir? >> >> I was thinking that if that existed we could build more controls on it >> based on Labels rather then just UIDs matching. > > Not sure if you are thinking of symlink attacks or hard link attacks. > SELinux supports preventing the former by restricting the ability to > follow symlinks based on lnk_file read permission, so you can prevent > trusted processes from following untrustworthy symlinks.  SELinux > supports preventing the latter by restricting the ability to create > hard links to unauthorized files.  But you need to write your policies > in a manner that leverages that support, and a fully unconfined domain > isn't going to be protected via SELinux by definition; ideally you'd > be phasing out unconfined altogether like Android did.  Modern kernels > also have the /proc/sys/fs/protected_hardlinks and > /proc/sys/fs/protected_symlinks settings, which restrict based on UID, > but the symlink checks aren't based on the target of the symlink either. Android does not have an Admin, so it is a lot easier for them.  But not going to get into that now.  I obviously understand how SELinux works.  But perhaps I am looking for something differntly. This link defines pretty close to what I would want, but extended for labels rather then just UIDS. https://sysctl-explorer.net/fs/protected_symlinks/ > A long-standing class of security issues is the symlink-based > time-of-check-time-of-use race, most commonly seen in world-writable > directories like /tmp. The common method of exploitation of this flaw > is to cross privilege boundaries when following a given symlink (i.e. > a **PRIVILEGED** process follows a symlink belonging **PROVIDED BY > OTHERS**). For a likely incomplete list of hundreds of examples across > the years, please see: > http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp > > When set to “0”, symlink following behavior is unrestricted. > > When set to “1” symlinks are permitted to be followed only when > outside a sticky world-writable directory **WE COULD POTENTIALLY SET > THIS OR SOME OTHER FLAG**, or when the **LABEL** of the symlink and > follower match, or when the directory **LABEL** matches the symlink’s > **LABEL**. > > This protection is based on the restrictions in Openwall and grsecurity. >