Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932181AbaJVSud (ORCPT ); Wed, 22 Oct 2014 14:50:33 -0400 Received: from mail-la0-f42.google.com ([209.85.215.42]:56187 "EHLO mail-la0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754676AbaJVSub (ORCPT ); Wed, 22 Oct 2014 14:50:31 -0400 MIME-Version: 1.0 In-Reply-To: References: <1413235430-22944-1-git-send-email-adityakali@google.com> <1413235430-22944-8-git-send-email-adityakali@google.com> <20141016211236.GA4308@mail.hallyn.com> <20141016214710.GA4759@mail.hallyn.com> <87iojgmy3o.fsf@x220.int.ebiederm.org> <44072106-c0f3-46b8-b2b5-9b1cbd1b7d88@email.android.com> <87zjcq10ya.fsf@x220.int.ebiederm.org> <87lhoayo59.fsf@x220.int.ebiederm.org> From: Andy Lutomirski Date: Wed, 22 Oct 2014 11:50:08 -0700 Message-ID: Subject: Re: [PATCHv1 7/8] cgroup: cgroup namespace setns support To: Aditya Kali Cc: "Eric W. Biederman" , "Serge E. Hallyn" , Linux API , Linux Containers , Serge Hallyn , "linux-kernel@vger.kernel.org" , Tejun Heo , cgroups@vger.kernel.org, Ingo Molnar Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 22, 2014 at 11:37 AM, Aditya Kali wrote: > On Tue, Oct 21, 2014 at 5:58 PM, Andy Lutomirski wrote: >> On Tue, Oct 21, 2014 at 5:46 PM, Aditya Kali wrote: >>> On Tue, Oct 21, 2014 at 3:42 PM, Andy Lutomirski wrote: >>>> On Tue, Oct 21, 2014 at 3:33 PM, Aditya Kali wrote: >>>>>> >>>>>>> And with explicit permission from >>>>>>> cgroup subsystem (something like cgroup.may_unshare as you had >>>>>>> suggested previously), we can make sure that unprivileged processes >>>>>>> cannot pin themselves. Also, maintaining this invariant (your current >>>>>>> cgroup is always under your cgroupns-root) keeps the code and the >>>>>>> semantics simple. >>>>>> >>>>>> I actually think it makes the semantics more complex. The less policy >>>>>> you stick in the kernel, the easier it is to understand the impact of >>>>>> that policy. >>>>>> >>>>> >>>>> My inclination is towards keeping things simpler - both in code as >>>>> well as in configuration. I agree that cgroupns might seem >>>>> "less-flexible", but in its current form, it encourages consistent >>>>> container configuration. If you have a process that needs to move >>>>> around between cgroups belonging to different containers, then that >>>>> process should probably not be inside any container's cgroup >>>>> namespace. Allowing that will just make the cgroup namespace >>>>> pretty-much meaningless. >>>> >>>> The problem with pinning is that preventing it causes problems >>>> (specifically, either something potentially complex and incompatible >>>> needs to be added or unprivileged processes will be able to pin >>>> themselves). >>>> >>>> Unless I'm missing something, a normal cgroupns user doesn't actually >>>> need kernel pinning support to effectively constrain its members' >>>> cgroups. >>>> >>> >>> So there are 2 scenarios to consider: >>> >>> We have 2 containers with cgroups: /container1 and /container2 >>> Assume process P is running under cgroupns-root '/container1' >>> >>> (1) process P wants to 'write' to cgroup.procs outside its >>> cgroupns-root (say to /container2/cgroup.procs) >> >> This, at least, doesn't have the problem with unprivileged processes >> pinning themselves. >> >>> (2) An admin process running in init_cgroup_ns (or any parent cgroupns >>> with cgroupns-root above /container1) wants to write pid of process P >>> to /container2/cgroup.procs (which lies outside of P's cgroupns-root) >>> >>> For (1), I think its ok to reject such a write. This is consistent >>> with the restriction in cgroup_file_write added in 'Patch 6' of this >>> set. I believe this should be independent of visibility of the cgroup >>> hierarchy for P. >>> >>> For (2), we may allow the write to succeed if we make sure that the >>> process doing the write is an admin process (with CAP_SYS_ADMIN in its >>> userns AND over P's cgroupns->user_ns). >> >> Why is its userns relevant? >> >> Why not just check whether the target cgroup is in the process doing >> the write's cgroupns? (NB: you need to check f_cred, here, not >> current_cred(), but that's orthogonal.) Then the policy becomes: no >> user of cgroupfs can move any process outside of the cgroupfs's user's >> cgroupns root. >> > Humm .. it doesn't have to be. I think its simpler to not enforce > artificial permission checks unless there is a security concern (and > in this case, there doesn't seem to be any). So I will leave the > capability check out from here. > >> I think I'm okay with this. >> >>> If this write succeeds, then: >>> (a) process P's /proc//cgroup does not show anything when viewed >>> by 'self' or any other process in P's cgrgroupns. I would really like >>> to avoid showing relative paths or paths outside the cgroupns-root >> >> The empty string seems just as problematic to me. > > Actually, there is no right answer here. Our options are: > * show relative path > -- this will break userspace as /proc//cgroup does not show > relative paths today. This is also very ambiguous (is it relative to > cgroupns-root or relative to /proc/cgroup file reader's cgroup?). > Confused now. If ".." in /proc/pid/group would be ambiguous, then so would a path relative to cgroupns root, right? Or am I missing something? (I'm not saying that ".." is beautiful or that it won't confuse things. I'm just not sure why it's ambiguous.) > * show absolute path > -- this will also wrong as the process won't be able to make sense of > it unless it has exposure to the global cgroup hierarchy. > -- worse case is this that the global path also exists under the > cgroupns-root ... so now the process thinks its in completely wrong > cgroup > -- this exposes system > > * show only "/" > -- this is arguably better, but if the process tires to verify that > its pid is in cgroup.procs of the cgroupns-root, its in for a > surprise! > > In either case, whatever we expose, the userspace won't be able to use > this path correctly (worse yet, it associates wrong cgroup for that > path). So I think its best to not print out the line for default > hierarchy at all. This happens today when cgroupfs is not mounted. I > am open to other suggestions. I suppose that ".." is a possible security problem. If I can force you to see lots of ..s in there, then I might be about to get you to write outside cgroupfs. Grr. No great solution here. I suppose that the empty string isn't so bad. We could also write something obviously invalid like "(unreachable)". As long as no one actually creates a cgroup called "(unreachable)", then this could result in errors but not actual confusion. >>> * should we then also allow setns() without first entering the >>> cgroupns-root? setns also checks the same conditions as in (a) plus it >>> checks that your current cgroup is descendant of target cgroupns-root. >>> Alternatively we can special-case setns() to own cgroupns so that it >>> doesn't fail. >> >> I think setns should completely ignore the caller's cgroup and should >> not change it. Userspace can do this. >> > > All above changes more or less means that tasks cannot pin themselves > by unsharing cgroupns. Do you agree that we don't need that "explicit > permission from cgroupfs" anymore (via cgroup.may_unshare file or > other mechanism)? Yes, I agree. > >>> * migration for these processes will be tricky, if not impossible. But >>> the admin trying to do this probably doesn't care about it or will >>> provision for it. >> >> Migration for processes in a mntns that have a current directory >> outside their mntns is also difficult or impossible. Same with >> pidnses with an fd pointing at /proc/self from an outside-the-pid-ns >> procfs. Nothing new here. >> >> --Andy > > Thanks for the review! No problem. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/