Re: [AppArmor 39/45] AppArmor: Profile loading and manipulation,pathname matching

From: david
Date: Mon Jun 11 2007 - 02:47:15 EST

On Sun, 10 Jun 2007, Crispin Cowan wrote:

Casey Schaufler wrote:
--- david@xxxxxxx wrote:

Yes, and in the process, AA stores compiled regular expressions in
kernel. Ouch. I'll take "each file it's own label" over _that_ any time.

and if each file has it's own label you are going to need regex or similar
to deal with them as well.

Now that you're going to have to explain. Nothing like that
on any of the MLS systems I'm familiar with, and I think that
I know just about all of them.

I suspect that David meant that if you were using "unique label per
file" as an implementation technique to implement AA on top of SELinux,
that you would then need a regexp to discern labels.


say that we give each file a unique label, and for simplicity we set the label == path (note that this raises the issue, what will SELinux do when there are multiple paths to the same file)

now say that you want to grant apache access to all files that have labels that follow the pattern '/home/*/http/* ?

you are either going to use regex matching, or you are going to have to enumerate every label that matches this (potentially a very large list). and if you try to generate the enumerated list you need to add a label to the list if a file is renamed or created to match the pattern, and delete a file from the list if it is renamed to no longer match the pattern

It's hard to recall with all the noise, but at this point in the thread
the discussion is about the best way to implement AA. Some have alleged
that AA layered on top of SELinux is the best way. I think that is
clearly wrong; AA layered on top of SELinux is possible, but would
require a bunch of enhancements to SELinux first, and the result would
be more complex than the proposed AA patch and have weaker functionality
and performance.

AA as-is needs to figure out how to deal with bind-mounts, and how to handle hardlink creation in a more ganular manner (and potentially other resources like network sockets), but it's useful now even without these improvements

AA over SELinux would need for SELinux to figure out how to handle file creation, file renames, and multiple paths for the same file (hard-links and bind-mounts). In addition a userspace daemon would have to be written to re-label files and/or change policy on the fly as files are renamed. the result would still have race conditions due to the need to re-label large numbers of files

ACPI should have taught everyone that sometimes putting an interpreter in the kernel really is the best option. looking at the problems of bouncing back out to userspace for file creation and renames it looks like a regex in the kernel is a lot safer and more reliable.

David Lang
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