Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756230AbbFRTFA (ORCPT ); Thu, 18 Jun 2015 15:05:00 -0400 Received: from gum.cmpxchg.org ([85.214.110.215]:44594 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755277AbbFRTEw (ORCPT ); Thu, 18 Jun 2015 15:04:52 -0400 Date: Thu, 18 Jun 2015 15:04:25 -0400 From: Johannes Weiner To: Tejun Heo Cc: lizefan@huawei.com, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH v2 3/4] cgroup: require write perm on common ancestor when moving processes on the default hierarchy Message-ID: <20150618190425.GB2182@cmpxchg.org> References: <1434481817-32001-1-git-send-email-tj@kernel.org> <1434481817-32001-4-git-send-email-tj@kernel.org> <20150618175927.GD12934@mtj.duckdns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150618175927.GD12934@mtj.duckdns.org> User-Agent: Mutt/1.5.23+89 (0255b37be491) (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2339 Lines: 48 On Thu, Jun 18, 2015 at 01:59:27PM -0400, Tejun Heo wrote: > On traditional hierarchies, if a task has write access to "tasks" or > "cgroup.procs" file of a cgroup and its euid agrees with the target, > it can move the target to the cgroup; however, consider the following > scenario. The owner of each cgroup is in the parentheses. > > R (root) - 0 (root) - 00 (user1) - 000 (user1) > | \ 001 (user1) > \ 1 (root) - 10 (user1) > > The subtrees of 00 and 10 are delegated to user1; however, while both > subtrees may belong to the same user, it is clear that the two > subtrees are to be isolated - they're under completely separate > resource limits imposed by 0 and 1, respectively. Note that 0 and 1 > aren't strictly necessary but added to ease illustrating the issue. > > If user1 is allowed to move processes between the two subtrees, the > intention of the hierarchy - keeping a given group of processes under > a subtree with certain resource restrictions while delegating > management of the subtree - can be circumvented by user1. > > This happens because migration permission check doesn't consider the > hierarchical nature of cgroups. To fix the issue, this patch adds an > extra permission requirement when userland tries to migrate a process > in the default hierarchy - the issuing task must have write access to > the common ancestor of "cgroup.procs" file of the ancestor in addition > to the destination's. > > Conceptually, the issuer must be able to move the target process from > the source cgroup to the common ancestor of source and destination > cgroups and then to the destination. As long as delegation is done in > a proper top-down way, this guarantees that a delegatee can't smuggle > processes across disjoint delegation domains. > > The next patch will add documentation on the delegation model on the > default hierarchy. > > v2: Fixed missing !ret test. Spotted by Li Zefan. > > Signed-off-by: Tejun Heo > Cc: Li Zefan Acked-by: Johannes Weiner -- 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/