[olug] SELINUX, you irritate me

Lou Duchez lou at paprikash.com
Tue Jul 16 15:04:08 CDT 2019

Yeah.  The trick is tuning it before activating it, rather than 
immediately activating it and then all aitch-eee-ell-ell breaks loose.

One thing SELinux does to earn my immediate ire is, first thing I do 
when I configure a new Linux server is switch SSH to a nondefault port.  
SELinux thwarts that every time, and that goes a long way to making me 
irrationally angry at SELinux.  There's a perfectly simple response to 
that -- throw SELinux into "permissive" mode and then do the audit2allow 
steps as outlined below (which will fix the SSH port thing) -- but 
SELinux does not really endear itself to me by making it harder to 
implement sensible security choices.

But that just means it's up to me to meet SELinux halfway.  Well, 99% of 
the way.

> It's a pain, but I'd rather have it than not.
> On Tue, Jul 16, 2019 at 2:44 PM Lou Duchez <lou at paprikash.com> wrote:
>> So when SELinux came out ages ago, I quickly developed a strong distaste
>> for it.  It felt like it was more likely to do harm than good, like a
>> security guard in your building who insists on quarantining your
>> groceries to make sure you're not a drug smuggler.  I found I had to
>> shut SELinux off to get things to work.
>> Years later, I am finally conceding that SELINUX is here to stay, and
>> it's time to learn to love it (or at least tolerate it).  Here's how I
>> finally got on SELinux's side:
>> 1)    Throw SELinux into "permissive" mode by editing
>> /etc/sysconfig/selinux and rebooting.  (If SELinux had been "disabled",
>> upon reboot, SELinux is going to have to relabel all the files on your
>> system.  This is actually pretty quick, unless you've got directories
>> and directories and directories of files.  I had to do this on a couple
>> servers that had years of daily snapshots on them, and after a couple
>> days the relabeling wasn't done.  I eventually deleted most of the old
>> backups -- kept one backup per month, and only since Jan 2018 -- and the
>> relabel took under 10 minutes.  Math-wise, I suspect the relabeling does
>> not scale linearly with the number of files, but perhaps with the square
>> of the number of files.)
>> 2)    After the reboot, you can run "audit2why -b" and "audit2allow -b"
>> to get information on opertaions that SELinux has noted have violated
>> policy since booting.  (There are options other than "-b", but I'm just
>> talking about how to make SELinux reasonable.  And to me, it's pretty
>> reasonable to look at how it's been doing since the last boot.)
>> 3)    You can run "audit2allow -b -M newrules" to create a file,
>> "newrules.pp", that contains SELinux rules necessary to allow all the
>> operations that were violating policy.  You can load it by running
>> "semodule -i newrules.pp".  You can also look at "newrules.te" to see a
>> more visually understandable list of new rules.  Now I won't claim to
>> fully understand what the rules are, but I can generally see processes I
>> recognize and take it on faith that they're trying to do something
>> reasonable.  Like recently I found this entry in newrules.te:
>>       allow dhcpd_t unlabeled_t:file { append getattr link map open read
>> unlink write };
>> I could do some digging to try to figure out exactly what file it's
>> trying to get at.  However, I also know that I've got some custom code
>> that creates and overwrites files in /var/lib/dhcpd, so it seems likely
>> that SELinux finds my custom code questionable.  Okay SELinux, you win,
>> I'll let you have that rule.
>> 4)    After applying new rules, reboot.  Maybe do another "audit2allow
>> -b" to see if anything is still coming up.
>> 5)    Every few days, see if SELinux is still coming up with messages
>> and warnings.  Hopefully you'll reach a  point where SELinux goes for
>> days without having any complaints.
>> 6)    Once you're satisfied that SELinux seems to be pretty happy with
>> things, THEN is when you switch SELinux to "enforcing", over in
>> /etc/sysconfig/selinux.
>> All that work to get SELinux properly tuned for your system.  But I ...
>> guess it makes things better?  People either love SELinux or hate it
>> with a passion, there seems to be no middle ground, and I think I see why.
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