From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935167AbeEYD5V (ORCPT ); Thu, 24 May 2018 23:57:21 -0400 Received: from ipmail07.adl2.internode.on.net ([150.101.137.131]:41806 "EHLO ipmail07.adl2.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934254AbeEYD5T (ORCPT ); Thu, 24 May 2018 23:57:19 -0400 Date: Fri, 25 May 2018 13:57:16 +1000 From: Dave Chinner To: "Eric W. Biederman" Cc: "Theodore Y. Ts'o" , Linux Containers , linux-fsdevel@vger.kernel.org, Seth Forshee , "Serge E. Hallyn" , Christian Brauner , linux-kernel@vger.kernel.org Subject: Re: [REVIEW][PATCH 0/6] Wrapping up the vfs support for unprivileged mounts Message-ID: <20180525035716.GE10363@dastard> References: <87o9h6554f.fsf@xmission.com> <20180524214617.GG7712@thunk.org> <87y3g8y6x9.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87y3g8y6x9.fsf@xmission.com> 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 On Thu, May 24, 2018 at 06:23:30PM -0500, Eric W. Biederman wrote: > "Theodore Y. Ts'o" writes: > > > On Wed, May 23, 2018 at 06:22:56PM -0500, Eric W. Biederman wrote: > >> > >> Very slowly the work has been progressing to ensure the vfs has the > >> necessary support for mounting filesystems without privilege. > > > > What's the thinking behind how system administrators and/or file > > systems would configure whether or not a particular file system type > > will be allowed to be mounted w/o privilege? > > The mechanism is .fs_flags in file_system_type. If the FS_USERNS_MOUNT > flag is set then root in a user namespace (AKA an unprivileged user) > will be allowed to mount to mount the filesystem. > > There are very real concerns about attacking a filesystem with an > invalid filesystem image, or by a malicious protocol speaker. So I > don't want to enable anything without the file system maintainers > consent and without a reasonable expecation that neither a system wide > denial of service attack nor a privilege escalation attack is possible > from if the filesystem is enabled. > > So at a practical level what we have in the vfs is the non-fuse specific > bits that enable unprivileged mounts of fuse. Things like handling > of unmapped uid and gids, how normally trusted xattrs are dealt with, > etc. > > A big practical one for me is that if either the uid or gid is not > mapped the vfs avoids writing to the inode. > > Right now my practical goal is to be able to say: "Go run your > filesystem in userspace with fuse if you want stronger security > guarantees." I think that will be enough to make removable media > reasonably safe from privilege escalation attacks. > > There is enough code in most filesystems that I don't know what our > chances of locking down very many of them are. But I figure a few more > of them are possible. I'm not sure we need to - fusefs-lkl gives users the ability to mount any of the kernel filesystems via fuse without us needing to support unprivileged kernel mounts for those filesystems. Cheers, Dave. -- Dave Chinner david@fromorbit.com