Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754088AbdFWQac (ORCPT ); Fri, 23 Jun 2017 12:30:32 -0400 Received: from h2.hallyn.com ([78.46.35.8]:57546 "EHLO h2.hallyn.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751620AbdFWQaa (ORCPT ); Fri, 23 Jun 2017 12:30:30 -0400 Date: Fri, 23 Jun 2017 11:30:30 -0500 From: "Serge E. Hallyn" To: Casey Schaufler Cc: "Serge E. Hallyn" , Amir Goldstein , Stefan Berger , "Eric W. Biederman" , Linux Containers , lkp@01.org, xiaolong.ye@intel.com, linux-kernel , Mimi Zohar , Tycho Andersen , James Bottomley , christian.brauner@mailbox.org, Vivek Goyal , LSM List Subject: Re: [PATCH 0/3] Enable namespaced file capabilities Message-ID: <20170623163030.GA18820@mail.hallyn.com> References: <1498157989-11814-1-git-send-email-stefanb@linux.vnet.ibm.com> <20170623160026.GA18257@mail.hallyn.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2711 Lines: 52 Quoting Casey Schaufler (casey@schaufler-ca.com): > On 6/23/2017 9:00 AM, Serge E. Hallyn wrote: > > Quoting Amir Goldstein (amir73il@gmail.com): > >> On Thu, Jun 22, 2017 at 9:59 PM, Stefan Berger > >> wrote: > >>> This series of patches primary goal is to enable file capabilities > >>> in user namespaces without affecting the file capabilities that are > >>> effective on the host. This is to prevent that any unprivileged user > >>> on the host maps his own uid to root in a private namespace, writes > >>> the xattr, and executes the file with privilege on the host. > >>> > >>> We achieve this goal by writing extended attributes with a different > >>> name when a user namespace is used. If for example the root user > >>> in a user namespace writes the security.capability xattr, the name > >>> of the xattr that is actually written is encoded as > >>> security.capability@uid=1000 for root mapped to uid 1000 on the host. > >>> When listing the xattrs on the host, the existing security.capability > >>> as well as the security.capability@uid=1000 will be shown. Inside the > >>> namespace only 'security.capability', with the value of > >>> security.capability@uid=1000, is visible. > >>> > >> Am I the only one who thinks that suffix is perhaps not the best grammar > >> to use for this namespace? > > You're the only one to have mentioned it so far. > > > >> xattrs are clearly namespaced by prefix, so it seems right to me to keep > >> it that way - define a new special xattr namespace "ns" and only if that > >> prefix exists, the @uid suffix will be parsed. > >> This could be either ns.security.capability@uid=1000 or > >> ns@uid=1000.security.capability. The latter seems more correct to me, > >> because then we will be able to namespace any xattr without having to > >> protect from "unprivileged xattr injection", i.e.: > >> setfattr -n "user.whatever.foo@uid=0" > > I like it for simplifying the parser code. One concern I have is that, > > since ns.* is currently not gated, one could write ns.* on an older > > kernel and then exploit it on a newer one. > > security.ns.capability@uid=1000, then? That loses the advantage of simpler parsing though. (Really it's not much of a simplification anyway.) So I'm not sure what advantage remains. > Or maybe just security.ns.capability, taking James' comment into account. That last one may be suitable as an option, useful for his particular (somewhat barbaric :) use case, but it's not ok for the general solution. If uid 1000 was delegated the subuids 100000-199999, it should be able to write a file capability for use by his subuids, but that file capability must not apply to other subuids. -serge