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Masochism never ends — Some improvements for Gentoo safety

Those of you who follow my blog for a long time might have already guessed what I’m going to talk about at this point: PAM. Each and every time I end up working on PAM I remind myself I’m probably a masochist in the deep of my soul, which is pretty bad by itself. I have to clear up though, that I’m not in the same rank as Max Mosley and that my masochism stop at maintaining packages that are ridiculously left alone by anybody else. Just to be safe, okay?

So what’s up in PAM land? Well, there are currently two things I’m experimenting with that I would like to see in our default PAM setup (through pambase) in the next months: capabilities and namespaces.

I already wrote something about capabilities in February 2008, but I never went around making sure it worked like it should. Right now I haven’t really even started the work to support that, but I really intend to get this working before the end of the year. This would complement eventual support for filesystem-based capabilities in Portage that I’ve already proposed a few times before. I got interested again in implementing that after reading that Fedora 12 is also doing so and that it would have solved the recent PulseAudio vulnerability (which luckily we addressed right away in another way).

For what concerns namespaces, instead, this is not only a security issue that I’m investigating but also a possible system setup improvement that could really be something. You might remember that some time ago I made it possible to quickly support pam_mktemp which creates per-user temporary directories in /tmp and make them inaccessible from other users.

Unfortunately there have been, with time, quite a few problems related to that functionality: at the time, start-stop-daemon in OpenRC didn’t really set up environment properly (and I think the same is also true for the version bundled with baselayout1), which in turn created problems to me and others using the daemon mode of emacs because it used the temporary directory to create a listening socket for the client to connect (and daemon and user ended up having different temporary directories). Nowadays, with the working command, and services using pam_mktemp, the problem appears with Samba: when starting Samba on a system set up with pam_mktemp, logging into shares that have usernames and passwords fail because Samba tries to use the user-defined temporary directory as the user “nobody” (which does not have the privilege to access that). Finally, lots of packages simply ignore the TMPDIR variable, either because they “want consistency” or because, like it’s the case for gnupg, upstream has no clue about Unix sockets and expects TMPDIR to mock stuff up via network, thus ignores it (breaching the GNU coding style guidelines, by the way).

So I decided to go with the alternative way, which is not supported on non-Linux systems (thus pam_mktemp is not going to die just yet), but is also available off the shelf on the Linux-PAM package: pam_namespace and polyinstantiated directories. To cut the story short and make it understandable to users, it simply means that once it’s set up, /tmp as seen by different users is a different directory altogether.

I’m still not sure if this is the way to go to be honest. When I first looked into it, it would break postgres, because the older settings would use /tmp to store the connection socket rather than /var/run; I also am not sure whether it’s totally fine in respect to security: since I usually start my services sudo’ing from my user, the directories used for polyinstantiation have “flame” as group; of course I cannot access them because I cannot access the base directory, but still… Also, this method seems to break the tramp feature of Emacs that allows to edit root-owned files from within an user session (it assumes that /tmp is always the same between users afaics).

Now, I said that namespaces are not only a security feature but can also help with configuration; some time ago I tried finding an easy way to set the XDG_CACHE_DIR variable for users so that instead of saving in ~/.cache it would use /var/cache (reason for this is that the home file system is usually smaller and should not be wasted for cache – in my case it’s also under RAID – while the /var/cache file system can just be cleaned up as needed). Unfortunately, our env-update command does not take it gently if hte $USER variable is used to define another variable, so I gave up. With pam_namespace, it would be possible to have a /var/cache/user directory that is an instance of /var/cache/users/$USER, and then have XDG_CACHE_DIR to use that directory. Similarly we could do that for /var/run/user so that local instance sockets are saved there rather than in /tmp (or /home) where they don’t belong.

So this is what is boiling in the PAM pot for the next months; you can always speed it up at your pleasure.

Comments 3
  1. Just as an head-up, pam_namespace got out of my radar for now, will write extensively later on why is that.

  2. Hi Diego,do I understand you right that /tmp should no longer be the same for all users? How should a heterogenous group of people on a server share file in this case? Home directories are not readable by others and /tmp seemed as a nice “Hey guys, take that file.” place.Thanks,Marcel

  3. Marcel, the temporary directory is not really the most suitable place to do that.While you shouldn’t give access to users’ home directories, you can create a sub-directory in there that is accessible by others. I do that to give files around to other developers in the machine.

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