Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1947165AbdDYTbc (ORCPT ); Tue, 25 Apr 2017 15:31:32 -0400 Received: from 251.110.2.81.in-addr.arpa ([81.2.110.251]:36418 "EHLO lxorguk.ukuu.org.uk" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1429543AbdDYTbL (ORCPT ); Tue, 25 Apr 2017 15:31:11 -0400 Date: Tue, 25 Apr 2017 20:30:15 +0100 From: One Thousand Gnomes To: Jann Horn Cc: Matt Brown , serge@hallyn.com, jmorris@namei.org, Greg Kroah-Hartman , jslaby@suse.com, Jonathan Corbet , Kees Cook , Andrew Morton , kernel-hardening@lists.openwall.com, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCH v5 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN Message-ID: <20170425203015.1ae62057@lxorguk.ukuu.org.uk> In-Reply-To: References: <20170425041512.13189-1-matt@nmatt.com> <20170425144746.0d8721a3@lxorguk.ukuu.org.uk> Organization: Intel Corporation X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2778 Lines: 56 On Tue, 25 Apr 2017 15:56:32 +0200 Jann Horn wrote: > On Tue, Apr 25, 2017 at 3:47 PM, Alan Cox wrote: > >> There could be a few user programs that would be effected by this > >> change. > >> See: > >> notable programs are: agetty, csh, xemacs and tcsh > >> > >> However, I still believe that this change is worth it given that the > >> Kconfig defaults to n. This will be a feature that is turned on for the > >> same reason that people activate it when using grsecurity. Users of this > >> opt-in feature will realize that they are choosing security over some OS > >> features > > > > Only in this case they are not. > > > > If I am at the point I have the ability to send you TIOCSTI you already > > lost because I can just open /dev/tty to get access to my controlling tty > > and use write(). > > In terms of PTYs, this patch does not try to prevent writes to a slave > device (which afaik is what /dev/tty will give you). It tries to prevent the > equivalent of writes to the master device. As far as I know, there is no > way to go from a slave to the corresponding master without having > access to the master in some other way already. Ok so the point I was trying to make about write and read is I can already trash your channel when you su. Probably less irritatingly. In the pty case yes I can go from the tty to pty trivially and then open it, however the owner of the pty side would normally have exclusivity so while it's a potential hole it isn't a trivial one. If I want to do the equvalent of the TIOCSTI attack then I fork a process and exit the parent. The child can now use ptrace to reprogram your shell to do whatever interesting things it likes (eg running child processes called "su" via a second pty/tty pair). Not exactly rocket science. The tty layer does not try to manage this because it can't and two processes with the same uid are not protected from one another in the traditional Unix model. As with anything else when you try and glue namespaces on top of a model not designed for it you get a pile of holes. There is no safe way to fix it if you can't trust the environment you are communicating through. Secure practice is either to make another connection or if local to switch console and use SAK then login. In the namespaces case it certainly makes sense to forbid a process in one namespace from typing characters into another namespace but to me that implies that tty sessions/job control are namespaced, and that makes transitioning namespace or even just typing stuff into a docker container shell rather more tricky to get right if you have to be in the right tty session _and_ namespace to use the tty. Alan