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

From: david
Date: Mon Jun 11 2007 - 05:35:39 EST

On Mon, 11 Jun 2007, Sean wrote:

david@xxxxxxx wrote:

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)

So don't do that then.

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

If AA requires regex matching in the kernel, perhaps it really isn't
appropriate for inclusion. Surely there has to be a better way than
requiring the kernel to do regex matches at runtime?

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

WRONG. The labels would be obtained from AA as needed, never recorded in
the file attributes. This would change nothing about what AA needed
to compute at runtime, just the way it implements the result.

Ok, you are proposing throwing out all the label handling that SELinux does, including any caching. forgive me if I agree with the SELinux people that this is a very bad idea.

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.

There hasn't yet been shown a requirement for a userspace daemon to implement
AA over SeLinux.

I thought the userspace component was what you were proposing instead of doing the regex matching in the kernel. if this isn't it what exactly are you proposing?

you don't want the regex matching in the kernel.

you don't want a userspace component to do the regex matching when files are created or renamed.

how exactly do you propose to figure out what should happen to a file when it is created or it (or a parent directory) is renamed?

AA policies are defined in terms of regex expressions. you say that this should be able to be done on top of SELinux somehow without changing the policies. so somewhere, something needs to interpret the regex to see if it matches the path. this needs to be either kernel code or userspace code. you have ruled out kernel code and are now claiming that userspace isn't needed.

David Lang
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