My free software activities, June 2017

  1. blog
  1. Debian Long Term Support (LTS)
    1. Mercurial remote code execution
    2. Puppet remote code execution
    3. Sudo privilege escalation
    4. Should we upload on fridays?
    5. Triage
  2. Other work
    1. Stressant
    2. Debiman
    3. Local server maintenance
    4. Docker and Subsonic
    5. Wallabako
    6. Small fry

Debian Long Term Support (LTS)

This is my monthly Debian LTS report. This time I worked on Mercurial, sudo and Puppet.

Mercurial remote code execution

I issued DLA-1005-1 to resolve problems with the hg server --stdio command that could be abused by "remote authenticated users to launch the Python debugger, and consequently execute arbitrary code, by using --debugger as a repository name" (CVE-2017-9462).

Backporting the patch was already a little tricky because, as is often the case in our line of work, the code had changed significantly in newer version. In particular, the commandline dispatcher had been refactored which made the patch non-trivial to port. On the other hand, mercurial has an extensive test suite which allowed me to make those patches in all confidence. I also backported a part of the test suite to detect certain failures better and to fix the output so that it matches the backported code. The test suite is slow, however, which meant slow progress when working on this package.

I also noticed a strange issue with the test suite: all hardlink operations would fail. Somehow it seems that my new sbuild setup doesn't support doing hardlinks. I ended up building a tarball schroot to build those types of packages, as it seems the issue is related to the use of overlayfs in sbuild. The odd part is my tests of overlayfs, following those instructions, show that it does support hardlinks, so there maybe something fishy here that I misunderstand.

This, however, allowed me to get a little more familiar with sbuild and the schroots. I also took this opportunity to optimize the builds by installing an apt-cacher-ng proxy to speed up builds, which will also be useful for regular system updates.

Puppet remote code execution

I have issued DLA-1012-1 to resolve a remote code execution attack against puppetmaster servers, from authenticated clients. To quote the advisory: "Versions of Puppet prior to 4.10.1 will deserialize data off the wire (from the agent to the server, in this case) with a attacker-specified format. This could be used to force YAML deserialization in an unsafe manner, which would lead to remote code execution."

The fix was non-trivial. Normally, this would have involved fixing the YAML parsing, but this was considered problematic because the ruby libraries themselves were vulnerable and it wasn't clear we could fix the problem completely by fixing YAML parsing. The update I proposed took the bold step of switching all clients to PSON and simply deny YAML parsing from the server. This means all clients need to be updated before the server can be updated, but thankfully, updated clients will run against an older server as well. Thanks to LeLutin at Koumbit for helping in testing patches to solve this issue.

Sudo privilege escalation

I have issued DLA-1011-1 to resolve an incomplete fix for a privilege escalation issue (CVE-2017-1000368 from CVE-2017-1000367). The backport was not quite trivial as the code had changed quite a lot since wheezy as well. Whereas mercurial's code was more complex, it's nice to see that sudo's code was actually simpler and more straightforward in newer versions, which is reassuring. I uploaded the packages for testing and uploaded them a year later.

I also took extra time to share the patch in the Debian bugtracker, so that people working on the issue in stable may benefit from the backported patch, if needed. One issue that came up during that work is that sudo doesn't have a test suite at all, so it is quite difficult to test changes and make sure they do not break anything.

Should we upload on fridays?

I brought up a discussion on the mailing list regarding uploads on fridays. With the sudo and puppet uploads pending, it felt really ... daring to upload both packages, on a friday. Years of sysadmin work hardwired me to be careful on fridays; as the saying goes: "don't deploy on a friday if you don't want to work on the weekend!"

Feedback was great, but I was surprised to find that most people are not worried worried about those issues. I have tried to counter some of the arguments that were brought up: I wonder if there could be a disconnection here between the package maintainer / programmer work and the sysadmin work that is at the receiving end of that work. Having myself to deal with broken updates in the past, I'm surprised this has never come up in the discussions yet, or that the response is so underwhelming.

So far, I'll try to balance the need for prompt security updates and the need for stable infrastructure. One does not, after all, go without the other...

Triage

I also did small fry triage:

Hopefully some of those will come to fruitition shortly.

Other work

My other work this month was a little all over the place.

Stressant

Uploaded a new release (0.4.1) of stressant to split up the documentation from the main package, as the main package was taking up too much space according to grml developers.

The release also introduces limited anonymity option, by blocking serial numbers display in the smartctl output.

Debiman

Also did some small followup on the debiman project to fix the FAQ links.

Local server maintenance

I upgraded my main server to Debian stretch. This generally went well, althought the upgrade itself took way more time than I would have liked (4 hours!). This is partly because I have a lot of cruft installed on the server, but also because of what I consider to be issues in the automation of major Debian upgrades. For example, I was prompted for changes in configuration files at seemingly random moments during the upgrade, and got different debconf prompts to answer. This should really be batched together, and unfortunately I had forgotten to use the home-made script I established when i was working at Koumbit which shortens the upgrade a bit.

I wish we would improve on our major upgrade mechanism. I documented possible solutions for this in the AutomatedUpgrade wiki page, but I'm not sure I see exactly where to go from here.

I had a few regressions after the upgrade:

Docker and Subsonic

I did my first (and late?) foray into Docker) and containers. My rationale was that I wanted to try out Subsonic, an impressive audio server which some friends have shown me. Since Subsonic is proprietary, I didn't want it to contaminate the rest of my server and it seemed like a great occasion to try out containers to keep things tidy. Containers may also allow me to transparently switch to the FLOSS fork LibreSonic once the trial period is over.

I have learned a lot and may write more about the details of that experience soon, for now you can look at the contributions I made to the unofficial Subsonic docker image, but also the LibreSonic one.

Since Subsonic also promotes album covers as first-class citizens, I used beets to download a lot of album covers, which was really nice. I look forward to using beets more, but first I'll need to implement two plugins.

Wallabako

I did a small release of wallabako to fix the build with the latest changes in the underlying wallabago library, which led me to ask upstream to make versionned releases.

I also looked into creating a separate documentation site but it looks like mkdocs doesn't like me very much: the table of contents is really ugly...

Small fry

That's about it! And that was supposed to be a slow month...

RSS
Subsonic is propietary
But it wasn't always so. There is libresonic that forked from the last free version. You might want to check it out.
Comment by Felipe —
comment 2
I knew about Libresonic! I even linked to it in the article, although maybe that wasn't very clear. I've updated the article to make a more obvious reference.
Comment by anarcat —
docker!
Docker is amazing! It's been like learning git for me; hard to wrap my head around at first but the more I know about it the more I can't imagine living without it.
Comment by mvc —
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