Lenny SE Linux on the Desktop


I have been asked about the current status of Lenny SE Linux on the Desktop.

The first thing to consider is the combinations of policies and configurations. I will number them if only for the purpose of this post, if the numbering is considered generally helpful it could be more widely adopted to describe configurations.

  1. Default configuration. This has the default policy and is configured with all users having the domain unconfined_t and daemons such as POP servers are allowed to access home directories of type unconfined_home_dir_t. This allows such daemons to attack privileged user accounts.
  2. Some restricted users. This is the same as above but with some users restricted. Daemons such as POP servers are only allowed to access the home directories of restricted users. This means that if a user is to have an unconfined account and receive email they must have two Unix accounts or receive their mail under /var/spool/mail. This is one setsebool command and one (or maybe a few) “semanage login -m” commands from the default configuration.
  3. All users restricted. The system administrator has the domain sysadm_t and users have domains such as user_t. This requires a few more semanage commands. It is equivalent to the old strict policy.
  4. MLS. This is anything that is based around the MLS policy.

Currently I have two Desktop machines running Lenny (a test machine and my EeePC) and one server. I have only just switched my test machine to enforcing mode so have no good data on it (apart from the fact that I can boot it up and login – which is always a good start). The server is running in permissive mode because I have not yet written the policy to allow the POP server to read from unconfined_home_dir_t. I could get it working by switching from level 1 to level 2 or 3, but I want to get level 1 server policy working for the benefit of others else first.

My EeePC however is fully functional, I have been doing some work on it – that mostly means running a ssh client under GNOME but that’s OK (desktop environments such as GNOME and KDE are quite complex and demanding, getting a machine to boot and run such a desktop environment tests out many parts of the system). It’s only at level 1 for the moment because I want to get level 1 working everywhere before moving to the higher levels. I want to get things ready for real users ASAP. With the way the policy is managed now it will be possible to move from level 1 to 2 or 3 without rebooting or interrupting running services. So once users have systems running well at level 1 they can easily increase the security at a later date.

The problems that I have had are due to text relocations in libraries (see my previous post about execmod permission [1]). I’ve filed bug report #493678 against libtheora0 [2] in regard to this issue and included a patch from Fedora (which disables the non-relocatable assembly code in question). It seems that upstream have some new assembler code to try and fix this issue, so hopefully we’ll have something that can make it into Lenny!

I’ve filed bug report #493705 against libswscale0 for the same issue [3]. I included a patch to turn off the assembler code in question but that was not well received. If anyone has some i386 assembler skill and some spare time I would appreciate it if you could try and find a way to make the code position independent while losing little or no performance.

One thing to note is that I am now using an Opteron 1212 (2.0GHz dual-core) system for compiling, I run the i386 DomU with a 64bit kernel (I expect that 32bit user-space runs faster with a 64bit kernel than a 32bit kernel), and the disks are reasonably fast. Even so it takes about 15 minutes to build libswscale0 and the other packages from the same source tree. Previously I was using a 1.0GHz Pentium-3 for my Lenny i386 development until I had the libswscale0 build process go for more than 90 minutes before running out of disk space! If your build machine is old enough to only be 32bit then you should probably plan on watching a movie or going to bed while the build is in progress.

I have built packages that work around the above bugs and included them in my Lenny repository [4]. If you take the packages from that repository plus the Lenny packages then you should have a functional desktop system at level 1. I would appreciate it if people would start testing that and providing feedback. One important issue is the discovery of libraries that want shared stacks, text relocations, and executable memory. The deadline for fixing them properly is even more of a problem due to the number of people who have to be involved in a solution (as compared to the policy where I can do it on my own).

One finally problem is a bug in xdm which causes it to give the wrong context for login sessions due to having an old version of the SE Linux related code [5]. Due to a combination of this and some policy bugs you can not login with xdm. This is not a hugely important issue as most people will use gdm (which has the newer patch) or kdm (which has no SE Linux patch but can use pam_selinux.so). Also another option is wdm which works with pam_selinux.so. I’ve had a response to my bug report suggesting that there’s a bug in the patch (which was taken from gdm so maybe there’s a bug in gdm code too). I haven’t responded to that yet as I’ve been concentrating on the things that will make the most impact for Lenny.

At this stage I’m still unsure of when the release team will cut me off and prevent further SE Linux related fixes from going in Lenny. I need at least one more update to the policy packages before Lenny is released. I could release one right now with some useful improvements over what is currently in unstable, but am waiting until I get some other things fixed.

If I get everything fully working at level 1 (both client and server) before Lenny then I will provide a similar status report for users and testers of levels 2 and 3. I don’t expect that I will even get a chance to test level 4 (MLS) properly before Lenny releases.

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2 thoughts on “Lenny SE Linux on the Desktop”

  1. uau says:

    You seem to downplay the importance of performance in FFmpeg in the bugreport. Compiling with -fPIC and disabling the asm which is not compatible with that will drop H264 decoding performance over 15% on x86. And there is lots of content for which most systems are not “already fast enough”. Only a minority of machines can play all high-definition H264 content without problems, and if a machine is running the x86 version instead of AMD64 then it most likely is not in that minority. Failing to play content in realtime usually makes it unwatchable in practice, so performance problems should not be considered less important than causing segfaults with the machine/video combinations in question.

    I consider it very unlikely that anyone would write PIC-compatible versions of all the FFmpeg asm before Lenny is released, at least not without unacceptably degrading performance.

    I’m not very familiar with SELinux, but if I understood correctly the problem with text relocations is you don’t want the executable to have the permissions necessary for them because an attacker could write and then execute code with those permissions. Could that be avoided by making the executable drop such permissions after doing the text relocations during initial linking? If the text relocations are done and permissions dropped before any outside data is accessed then there should not be much risk of an attacker being able to exploit them. Unless there is some fundamental reason why such a system wouldn’t work implementing it sounds more realistic than making all code on x86 work with a register less.

  2. Elmar Hoffmann says:

    I’m only starting to look at SELinux, so I might have missed something there, but:
    Wouldn’t a configuration where POP and IMAP daemons are allowed access to mail-specific subdirectories of user home directiories (such as ~/Maildir) be much better than both 1 or 2 in this case?
    Better than 1 from a principle of least privilege perspective (and also better than 2 if the restricted users have anything else than mail in their homedir), and better than 2 from a usability perspective not requiring an extra set of accounts.

    BTW, scanelf(1) from the pax-utils package can be used to easily find shared libs with text relocations on a system, e.g. like this:
    scanelf –quiet –textrel –recursive /lib /usr/lib

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