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=-3.8 required=3.0 tests=BAYES_00, 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 7F18EC433DF for ; Wed, 14 Oct 2020 19:46:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3231B214D8 for ; Wed, 14 Oct 2020 19:46:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727445AbgJNTqh (ORCPT ); Wed, 14 Oct 2020 15:46:37 -0400 Received: from out02.mta.xmission.com ([166.70.13.232]:44598 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726111AbgJNTqg (ORCPT ); Wed, 14 Oct 2020 15:46:36 -0400 Received: from in02.mta.xmission.com ([166.70.13.52]) by out02.mta.xmission.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1kSmjE-00CGrH-J5; Wed, 14 Oct 2020 13:46:28 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1kSmjD-0008OD-AJ; Wed, 14 Oct 2020 13:46:28 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: "Serge E. Hallyn" Cc: Andy Lutomirski , Josh Triplett , Christian Brauner , Linux Containers , Alexander Mihalicyn , Mrunal Patel , Wat Lim , Aleksa Sarai , Pavel Tikhomirov , Geoffrey Thomas , Joseph Christopher Sible , =?utf-8?Q?Micka=C3=ABl_Sal?= =?utf-8?Q?a=C3=BCn?= , Vivek Goyal , Giuseppe Scrivano , Stephane Graber , Kees Cook , Sargun Dhillon , LKML References: <20200830143959.rhosiunyz5yqbr35@wittgenstein> <20201010042606.GA30062@mail.hallyn.com> <20201011205306.GC17441@localhost> <87h7r0qbqi.fsf@x220.int.ebiederm.org> <20201012150006.GA3503@mail.hallyn.com> Date: Wed, 14 Oct 2020 14:46:46 -0500 In-Reply-To: <20201012150006.GA3503@mail.hallyn.com> (Serge E. Hallyn's message of "Mon, 12 Oct 2020 10:00:06 -0500") Message-ID: <87wnzsmvyx.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1kSmjD-0008OD-AJ;;;mid=<87wnzsmvyx.fsf@x220.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/dzA0ojHgVFG1accwEPvk6reMlSI0NUgI= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org "Serge E. Hallyn" writes: > On Mon, Oct 12, 2020 at 12:01:09AM -0500, Eric W. Biederman wrote: >> Andy Lutomirski writes: >> >> > On Sun, Oct 11, 2020 at 1:53 PM Josh Triplett wrote: >> >> >> >> On Fri, Oct 09, 2020 at 11:26:06PM -0500, Serge E. Hallyn wrote: >> >> > > 3. Find a way to allow setgroups() in a user namespace while keeping >> >> > > in mind the case of groups used for negative access control. >> >> > > This was suggested by Josh Triplett and Geoffrey Thomas. Their idea was to >> >> > > investigate adding a prctl() to allow setgroups() to be called in a user >> >> > > namespace at the cost of restricting paths to the most restrictive >> >> > > permission. So if something is 0707 it needs to be treated as if it's 0000 >> >> > > even though the caller is not in its owning group which is used for negative >> >> > > access control (how these new semantics will interact with ACLs will also >> >> > > need to be looked into). >> >> > >> >> > I should probably think this through more, but for this problem, would it >> >> > not suffice to add a new prevgroups grouplist to the struct cred, maybe >> >> > struct group_info *locked_groups, and every time an unprivileged task creates >> >> > a new user namespace, add all its current groups to this list? >> >> >> >> So, effectively, you would be allowed to drop permissions, but >> >> locked_groups would still be checked for restrictions? >> >> >> >> That seems like it'd introduce a new level of complexity (a new facet of >> >> permission) to manage. Not opposed, but it does seem more complex than >> >> just opting out of using groups for negative permissions. > > Yeah, it would, but I basically hoped that we could catch most of this at > e.g. generic_permission(), and/or we could introduce a helper which > automatically adds a check for permission denied from locked_groups, so > it shouldn't be too wide-spread. If it does end up showing up all over > the place, then that's a good reason not to do this. > >> > Is there any context other than regular UNIX DAC in which groups can >> > act as negative permissions or is this literally just an issue for >> > files with a more restrictive group mode than other mode? >> >> Just that. >> >> The ideas kicked around in the conversation were some variant of having >> a sysctl that says "This system never uses groups for negative >> permissions". >> >> It was also suggested that if the sysctl was set the the permission >> checks would be altered such that even if someone tried to set a >> negative permission, the more liberal permissions of other would be used >> instead. > > So then this would touch all the same code points which the > locked_groups approach would have to touch? No locked_groups would touch in_group_p and set_groups. Especially what set_groups means in that context. It would have to handle what happens when you start accumulating locked groups (because of multiple namespaces). How you dedup locked groups etc. I was not able to convince myself that not being able to clear out groups that a user has when they create a user namespace won't cause other problems. Especially as user namespaces had been in use for a while at that point. Not supporting negative groups would touch acl_permission and modify it like: static int acl_permission_check(struct inode *inode, int mask) { [irrelveant code snipped] /* Only RWX matters for group/other mode bits */ mask &= 7; /* * Are the group permissions different from * the other permissions in the bits we care * about? Need to check group ownership if so. */ if (mask & (mode ^ (mode >> 3))) { - if (in_group_p(inode->i_gid)) + if (in_group_p(inode->i_gid) && + (!sysctl_force_positive_groups || + (mask & ~(mode >> 3))) mode >>= 3; } /* Bits in 'mode' clear that we require? */ return (mask & ~mode) ? -EACCES : 0; } I don't know that we need to do that. But it would might be a good way of flushing out the issues. >> Given that creating /etc/subgid is effectively opting out of negative >> permissions already have a sysctl that says that upfront feels like a >> very clean solution. >> >> Eric > > That feels like a cop-out to me. If some young admin at Roxxon Corp decides > she needs to run a container, so installs subuid package and sets that sysctl, > how does she know whether or not some previous admin, who has since retired and > did not keep good docs, set things up so that a negative acl is keeping nginx > from reading some supersecret doc? > > Now personally I'm not a great believer in the negative acls so I think the > above is a very unlikely scenario, but if we're going to worry about it, then > we should worry about it :) There is a different between guaranting we don't break existing setups when a new feature is enabled, and supporting old very rare setups when a new feature is enabled. > "Click this button if noone has ever used feature X on this server" My current thinking is that if we already don't honor negative groups when /etc/subgid exists it would not hurt to make that more explicit. >From what we could tell at the time people that know negative groups are honored much less systems that actually use negative groups are exceedingly rare. Eric