Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161392AbdDZOWg (ORCPT ); Wed, 26 Apr 2017 10:22:36 -0400 Received: from relay8-d.mail.gandi.net ([217.70.183.201]:53027 "EHLO relay8-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1039802AbdDZOWQ (ORCPT ); Wed, 26 Apr 2017 10:22:16 -0400 X-Originating-IP: 72.66.113.207 Subject: Re: [PATCH v5 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN To: One Thousand Gnomes , Jann Horn References: <20170425041512.13189-1-matt@nmatt.com> <20170425144746.0d8721a3@lxorguk.ukuu.org.uk> <20170425203015.1ae62057@lxorguk.ukuu.org.uk> <20170425222135.2859b1a8@lxorguk.ukuu.org.uk> <20170426134732.7579ebdd@lxorguk.ukuu.org.uk> Cc: 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 From: Matt Brown Message-ID: <07bec2b1-c945-fc6e-0238-28acb21b5cfb@nmatt.com> Date: Wed, 26 Apr 2017 10:21:36 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170426134732.7579ebdd@lxorguk.ukuu.org.uk> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2949 Lines: 66 On 04/26/2017 08:47 AM, One Thousand Gnomes wrote: >> open() what? As far as I know, for System-V PTYs, there is no path you can >> open() that will give you the PTY master. Am I missing something? > > Sorry brain fade - no. >> >>>>> 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. >>>> >>>> Why would the child be able to ptrace the shell? AFAICS, in the most >>>> relevant scenarios, the child can't ptrace the shell because the >>>> shell has a different UID (in the case of e.g. su or sudo). In other >>> >>> If I am the attacker wanting to type something into your su when you go >>> and su from my account, or where the user account is trojanned I do the >>> following >>> >>> fork >>> exit parent >>> child ptraces the shell (same uid as it's not setuid) >>> >>> You type "su" return >>> The modified shell opens a new pty/tty pair and runs su over it >>> My ptrace hooks watch the pty/tty traffic until you go to the loo >>> My ptrace hooks switch the console >>> My ptrace hooks type lots of stuff and hack your machine while eating the >>> output >>> >>> and you come back, do stuff and then exit >>> >>> And if you are in X it's even easier and I don't even need to care about >>> sessions or anything. X has no mechanism to sanely fix the problem, but >>> Wayland does. >> >> I think the "When using a program like su or sudo" in the patch description >> refers to the usecase where you go from a more privileged context (e.g. a >> root shell) to a less privileged one (e.g. a shell as a service-specific >> account used to run a daemon), not the other way around. > > Which is the sudo case and why sudo uses a separate pty/tty pair as it's > not just TIOCSTI that's an issue but there are a load of ioctls that do > things like cause signals to the process or are just annoying - > vhangup(), changing the speed etc > > (And for console changing the keymap - which is a nasty one) > Are any of these annoyances potential security issues? I would be happy to add patches or modify this one to include extra hardening measures. >> [However, I do think that it's a nice side effect of this patch that it will >> prevent a malicious program from directly injecting something like an >> SSH command into my shell in a sufficiently hardened environment >> (with LSM restrictions that prevent the malicious program from opening >> SSH keyfiles or executing another program that can do that). Although >> you could argue that in such a case, the LSM should be taking care of >> blocking TIOCSTI.] > > I would submit that creating a new pty/tty pair is the proper answer for > that case however. Making the tty calls respect namespaces is however > still a no-brainer IMHO. > > Alan >