Security mitigation strategies or, the only secure computer is the one yet to assemble.

Short preamble: I’m in a very depressed mood, like I haven’t been in month; this is very bad for my health but usually means I can focus on things much better, so you might actually find out I’m doing more than usual. Of course there is also to count in that I’m working during holidays so it’s not going to be all nice at all, even counting my depression off.

As I’ve written, I don’t trust closed-source software even the slightest; even though it does not really mean that free software is much better, process-wise, dealing with bundled libraries (like the bundled libs bug shows), with free software, or at least open-source software, there is the chance to check the sources out to fix the eventual issues.

This means that I won’t be using closed source software where security is a major concern, but since sometimes I have to use closed-source software, like Skype, or Sun’s compiler, it’s obvious that I have to find a compromise so I can still use them and yet feel reasonably safe. This is what is usually called having a mitigation strategy.

One of the most complex and well known mitigation strategies is of course SElinux, which makes a Linux system more like an APC than a computer. But such a system is probably safe to consider overkill for most systems, especially power user desktop systems.

Since this is, as I said, overkill, I’m more prone to look at smaller strategies, one of which I already discussed about: pam_mktemp . This module allows to create per-user private directories that make it much harder to exploit insecure temporary files vulnerabilities. Which is very nice since this seems to be a very common class of vulnerabilities, and my data shows that there is way too much software that still uses insecure functions to create temporary files, closed and open source alike.

Unfortunately, as you can read in my earlier blog post, this is not automatically a way out of the problem. The start-stop-daemon command from OpenRC plays nice with this just in the last release, and even with that, there are problems. The first problem is that the way pam_mktemp works, there is a need for the software calling PAM to open the session to properly set up the environment with its changes (which is what s-s-d lacked in previous versions). This causes for instance the gnome-keyring daemon to start with the wrong temporary directory when started by the PAM session chain. Even though pam_mktemp is invoked before the daemon, by the time it’s started the TMPDIR variable is not set in the environment. The reason for this is that the variable should not be changed if the session chain aborts the login.

The second problem is that not all software supports TMPDIR properly; Emacs has been fixed recently and now the emacs daemon starts up properly, but other software ignores TMPDIR altogether. VirtualBox (of which I still have things to say beside this) does not respect it for instance, which means that the module wouldn’t have spared you from the recent vulnerability that involved the software.

The third problem is that sometimes software expects TMPDIR to be world-readable, which is a bad assumption; Samba does this, and since s-s-d is now fixed, it now fails to work on my system. I still haven’t found out whether the PAM session chain was called at that point, and it’s just duplicating the problem with s-s-d with a different symptom, or if it fails to call it entirely. In either case, it’s a thing that has to be fixed to make sure that mitigation strategies like this one get in the default spirit of users.

But again this is just one part of the problem, and one part of mitigation. Other problems relate to the way we run some of the services, a lot of which still run as root rather than under a unprivileged user; while the git-daemon issue is now solved and the default install does not run as root any longer, there are more daemons that have the same problems.

Just as an example, I noticed that the iSCSI daemon ietd still runs under root, and I’ve added that to the list of software I have to check to see if I can improve it. Similarly, the init script for mpd does not use s-s-d to switch user but leaves it to mpd itself, spawning it by default with unneeded root privileges, and additionally not allowing pam_mktemp to create a new temporary directory for the mpd user (I have to spend some time on that since I’d also like to provide an alternative init script with multiplexing, which would then allow to run multiple mpds for different users, and in my case to just have the single mpd running as my own user rather than a different user entirely).

At any rate, I’m going to continue my best to make sure that secure defaults are in place in Gentoo, and that further mitigation strategies can be made available so that the users forced to use proprietary closed-source software don’t need to just accept whatever comes their way. Please join my efforts, if you can, by checking which software ignores TMPDIR and asking nicely upstream to fix the issue.