From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759075AbaKUQop (ORCPT ); Fri, 21 Nov 2014 11:44:45 -0500 Received: from mail-oi0-f51.google.com ([209.85.218.51]:37732 "EHLO mail-oi0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758856AbaKUQon (ORCPT ); Fri, 21 Nov 2014 11:44:43 -0500 Date: Fri, 21 Nov 2014 10:44:41 -0600 From: Seth Forshee To: Miklos Szeredi Cc: "Serge E. Hallyn" , "Eric W. Biederman" , "Serge H. Hallyn" , Andy Lutomirski , Michael j Theall , fuse-devel , Kernel Mailing List , Linux-Fsdevel Subject: Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user_ns Message-ID: <20141121164441.GA1730@ubuntu-mba51> References: <1414013060-137148-1-git-send-email-seth.forshee@canonical.com> <1414013060-137148-3-git-send-email-seth.forshee@canonical.com> <20141111140454.GD333@tucsk> <87mw7xd9zt.fsf@x220.int.ebiederm.org> <20141112130915.GG333@tucsk> <20141112162254.GB31775@ubuntu-hedt> <20141118152156.GA21726@ubuntu-mba51> <20141119140911.GA27009@mail.hallyn.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141119140911.GA27009@mail.hallyn.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 19, 2014 at 03:09:11PM +0100, Serge E. Hallyn wrote: > Quoting Miklos Szeredi (miklos@szeredi.hu): > > On Wed, Nov 19, 2014 at 9:50 AM, Miklos Szeredi wrote: > > > On Tue, Nov 18, 2014 at 4:21 PM, Seth Forshee > > > wrote: > > >>> I asked around a bit, and it turns out there are use cases for nested > > >> containers (i.e. a container within a container) where the rootfs for > > >> the outer container mounts a filesystem containing the rootfs for the > > >> inner container. If that mount is nosuid then suid utilities like ping > > >> aren't going to work in the inner container. > > >> > > >> So since there's a use case for suid in a userns mount and we have what > > >> we belive are sufficient protections against using this as a vector to > > >> get privileges outside the container, I'm planning to move ahead without > > >> the MNT_NOSUID restriction. Any objections? > > > > > > In the general case how'd we prevent suid executable being tricked to > > > do something it shouldn't do by unprivileged mounting into sensitive > > > places (i.e. config files) inside the container? > > The design of the namespaces would prevent that. You cannot manipulate your > mounts namespace unless you own it. You cannot manipulate the mounts namespace > for a task whose user namespace you do not own. If you can, for instance, > bind mount $HOME/shadow onto /etc/shadow, then you already own your user > namespace and are root there, so any suid-root program which you mount through > fuse will only subjegate your own namespace. Any task which running in the > parent user-ns (and therefore parent mount-ns) will not see your bind mount. > > > > Allowing SUID looks like a slippery slope to me. And there are plenty > > > of solutions to the "ping" problem, AFAICS, that don't involve the > > > suid bit. > > > > ping isn't even suid on my system, it has security.capability xattr instead. > > security.capability xattrs that will have the exact same concerns wrt > confusion through bind mounts as suid. > > > Please just get rid of SUID/SGID. It's a legacy, it's a hack, not > > worth the complexity and potential problems arising from that > > complexity. > > Oh boy, I don't know which side to sit on here :) I'm all for replacing > suid with some use of file capabilities, but realistically there are reasons > why that hasn't happened more widely than it has - tar, package managers, > cpio, nfs, etc. Miklos: I we're all generally in agreement here that suid/sgid is not the best solution, but as Serge points out we are unfortunately not yet in a place where it can be completely dropped in favor of capabilities. In light of this can I convince you to reconsider your position? Thanks, Seth