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=-0.8 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 58D74C31E41 for ; Mon, 10 Jun 2019 14:38:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2B4C320679 for ; Mon, 10 Jun 2019 14:38:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390835AbfFJOiG (ORCPT ); Mon, 10 Jun 2019 10:38:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47412 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388932AbfFJOiG (ORCPT ); Mon, 10 Jun 2019 10:38:06 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D3323308424C; Mon, 10 Jun 2019 14:38:00 +0000 (UTC) Received: from localhost.localdomain (ovpn-112-64.rdu2.redhat.com [10.10.112.64]) by smtp.corp.redhat.com (Postfix) with ESMTP id BF2DA608CD; Mon, 10 Jun 2019 14:37:58 +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> <3957e38f-e62b-9800-da58-33ccb351a849@redhat.com> <53479d32-a04d-2217-c4dd-9ce34bbba8ef@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: Date: Mon, 10 Jun 2019 10:37:57 -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: <53479d32-a04d-2217-c4dd-9ce34bbba8ef@tycho.nsa.gov> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Mon, 10 Jun 2019 14:38:05 +0000 (UTC) Sender: selinux-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: selinux@vger.kernel.org On 6/10/19 10:08 AM, Stephen Smalley wrote: > On 6/8/19 10:08 AM, Daniel Walsh wrote: >> 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. >>> > > That's the /proc/sys/fs/protected_symlinks feature I mentioned in my > email above.  It isn't based on the target of the symlink; it is only > based on the attributes of the follower process (e.g. root), the > attributes of the parent directory containing the symlink (e.g. /tmp), > and the attributes of the symlink file (e.g. /tmp/foo -> /etc/shadow). > At no point is it checking anything about the target of the symlink, > e.g. /etc/shadow.  If dwalsh creates a symlink under /tmp (ln -s > /etc/shadow /tmp/foo) and root tries to follow /tmp/foo, then that > will fail because 1) the process fsuid (root) != the /tmp/foo symlink > owner (dwalsh), and 2) /tmp is a sticky and world-writable directory, > and 3) the /tmp directory owner (root) != the /tmp/foo symlink owner > (dwalsh). Note that conditions (2) and (3) render the check useless > for your use case, since you want to prevent following any symlinks > writable by container processes in any directory within the container > filesystem, so the directory need not be world-writable/sticky and the > parent directory UID/label might be identical to the symlink UID/label. We we are mounting the file system (Most of the time), So we could add a flag to indicate that this is a protected file system. > > > The existing SELinux lnk_file read permission check enables you to > apply stronger label-based controls to all symlinks within the > container filesystem, not just ones in /tmp-like directories.  Don't > allow unconfined_t or any other privileged domain read permission to > container_file_t:lnk_file (or preferably to any file type for which > :lnk_file create is allowed to container process domains), and you'll > never have to worry about them following a symlink writable by a > container process.  This of course assumes that the container > filesystem is always labeled with a type that is untrusted, whether > via mount contexts or actual labels. But we want to allow domains to follow container_file_t links that point to container_file_t objects.  Just not follow them if they point to other types.  This means there is no Protection that I could write to a domain like unconfined_t to say only follow links when the types match.  Or the types have allow rules.