Previously I wrote about SELinux being bad for security on the grounds that it consumed too much of your time for little benefit. But I recently discovered it actively undermining the security on hosts I am responsible for.
The biggest wins for security are education and regular patching. So ensuring there is a patching mechanism is at the top of my list when building a new host. While the advice from NIST still reads like something written in the 1970s, NCSC and others endorse automated patching. I've not have any issues with Ubuntu/Debian across hundreds of machines running automated patching for several years - but I'm still cautious about how this is implemented.
I recently inherited some Centos 7 boxes which did not have automatic patching. While there is a RHEL package yum-cron this is quite a history of issues. I would also need to break the packaged files to ensure that my live boxes did not update at the same time as the dev/test. I appreciate that delaying the updates to live might still mean a different patch set gets deployed than is on dev/test....but it is a risk which can be reduced. So I just added a simple cron job to run `yum -y update` logging its stdout and stderr.
I thought all was good. Cron job was firing. Yum was happily reporting that there was nothing to do. Job done. What's next?
Only it was not working.
Updates were available. They were not installed. Yum said "No packages marked for update".
Running an update via an interactive session worked fine.
The culprit? SELinux.
It took a bit of experimentation to find this. On a dev system, sitting idle (other than my ssh session) SELinux was generating so much noise that the default log rotation config only allowed for 40 minutes of history. Setting up a more frequent cron job I was able to capture failures in the audit.log. So yum was being blocked by SELinux but reporting no errors. In fairness, the absence of errors from yum was not the fault of SELinux....however yum is now produced/maintained by RedHat.
OK....so maybe there was a reason for installing yum-cron after all? So I installed that (I do find it surprising that it EXPECTS manual intervention before it will actually work). Configured it. And nothing. `yum list updates` says there are lots of packages to update. The cron job fires. Yum-cron writes no logs. No updates installed. I amended the yum-cron config to send an email when patching has been attempted - will monitor and see how it goes.
This was an interesting journey. In addition to the issues with RHEL yum-cron, I discovered that Centos yum-cron has issues all of its own.
So to summarize....
- SELinux blocks yum run from cron
- yum does not report failures
- Centos yum-cron can't install security fixes only
