On 29Mar2019 1218, Christian Heimes wrote:
On 28/03/2019 23.35, Steve Dower wrote:
The ``importlib.util.open_for_import()`` function is a drop-in replacement for ``open(str(pathlike), 'rb')``. Its default behaviour is to open a file for raw, binary access. To change the behaviour a new handler should be set. Handler functions only accept ``str`` arguments. The C API ``PyImport_OpenForImport`` function assumes UTF-8 encoding.
All import and execution functionality involving code from a file will be changed to use ``open_for_import()`` unconditionally. It is important to note that calls to ``compile()``, ``exec()`` and ``eval()`` do not go through this function - an audit hook that includes the code from these calls is the best opportunity to validate code that is read from the file. Given the current decoupling between import and execution in Python, most imported code will go through both ``open_for_import()`` and the log hook for ``compile``, and so care should be taken to avoid repeating verification steps.
There is no Python API provided for changing the open hook. To modify import behavior from Python code, use the existing functionality provided by ``importlib``.
I think that the import hook needs to be extended. It only works for simple Python files or pyc files. There are at least two other important scenarios: zipimport and shared libraries.
For example how does the importhook work in regarding of alternative importers like zipimport? What does the import hook 'see' for an import from a zipfile?
Yes, good point. I think opening the zip file with open_for_import() is the right place to do it, as this operation relates to opening the file on disk rather than files within it.
Shared libraries are trickier. libc doesn't define a way to dlopen() from a file descriptor. dlopen() takes a file name, but a file name leaves the audit hook open to a TOCTOU attack.
For Windows, at least, the operating system can run its own validation on native modules (if you're using a feature like DeviceGuard, for example), so the hook likely isn't necessary for those purposes. I believe some configurations of Linux allow this as well?
But there's likely no better option here than a combination of good ACLs and checking by filename, which at least lets you whitelist the files you know you want to allow. Similarly for the zip file - if you trust a particular file and trust your ACLs, checking by filename is fine. That said, specific audit events for "I'm about to open this zip/dlopen this file for import" are very easy to add. (The PEP proposes many examples, but is not trying to be exhaustive. If accepted, we should feel free to add new events as we identify places where they matter.)
Aside: an important aspect of this per-file approach to execution is that the idea is generally to *enable* the files you trust, rather than disable the files that are bad. So the detection routines are typically "does this match a known hash" or "is this in a secure location", which for a carefully deployed system are already known values, rather than trying to figure out whether a file might do a bad thing. If you can't validate the files in your deployment match the ones you thought you were deploying, you are so far from needing this that it doesn't even matter, but most of the deployments I work with are *at least* this well controlled.