Monkeypatches to minimize the permissions required to run python under AppArmor
Monkeypatches to minimize the permissions required to run python under AppArmor.
What does this do?
Imagine you’ve written a simple Django application. Maybe you just followed the Django tutorial. All your code does is a bit of database querying. Then you deploy it under AppArmor and:
type=AVC msg=audit(1443087838.797:1078): apparmor="DENIED" operation="exec" profile="helloworld-application" name="/bin/dash" pid=8202 comm="python" requested_mask="x" denied_mask="x" fsuid=999 ouid=0
Suddenly your audit log is full of messages complaining that your application is trying to run a shell. You certainly didn’t write code to do that. Whats going on? Did you get hacked?
You (probably ;)) didn’t get hacked.
Turns out python shells out when doing some fairly mundane stuff. Up until now you could:
- Ignore it. Let your audit log have DENIED entries that you have to ignore. Now it’s hard to spot suspicious behaviour in your monitoring. Thinks might be broken, thinks might not be broken. Hope you don’t have to go through that log with a security professional.
- Allow it. Now your profile is broader than it needs to be. You are throwing away the security you gain in the first place. Hope you don’t have to go through that profile with a security professional.
This package patches several stdlib API’s to avoid subprocess usage, letting you keep your simple profiles and clean audit logs.
ctypes vs ldconfig
One of the first things I caught by application doing was trying to run gcc. This turned out to be a fallback for when an earlier attempt to run ldconfig had failed.
This turned out to be how ctypes.util.find_library works. This can be used in a few places:
- Gunicorn uses it for its sendfile implementation.
- Python’s uuid module uses for uuid4. Just importing the uuid module triggers this, even if you aren’t using uuid4.
platform.uname vs os.uname
platform.uname is mostly the same as os.uname, but there is an extra field. The field is sourced by shelling out and running uname -p:
sh -c "uname %s 2>%s"
This is used in several places:
- A command trick for getting your own version number is pkg_resources.require('myapp').version, which triggers it.
- Gunicorn triggers it via a platform.system() in gunicorn.workers.workertmp before it even loads your code.
Activating the monkey patches
As early in your code as possible do:
from apparmor_monkeys import patch_modules patch_modules()
Automatically via .pth hooks
And site-packages directory is scanned for .pth files. These are processed in order and are generally just a list of paths to add to sys.path. However any import lines will be honoured.
Create an apparmor-monkeys.pth in your virtualenvs site-packages directory containing:
import apparmor_monkeys; apparmor_monkeys.patch_modules()
It must be on a single line for this trick to work.
You can harden your AppArmor profiles further using change_profile to switch into a different profile after initialising your app.
If using multiprocess gunicorn (i.e. synchronous gunicorn) then you can wrap your workers in their own specific profile. In your gunicorn config you can add a hook to do this:
from apparmor_monkeys import change_profile def post_fork(server, worker): change_profile("myapplication//worker")
You can do this for celery too:
from apparmor_monkeys import change_profile from celery import signals @signals.worker_process_init.connect def switch_apparmor_profile(sender=None, signal=None): change_profile("tenselfservice-worker//worker")