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SubjectRe: [AppArmor 39/45] AppArmor: Profile loading and manipulation, pathname matching
On Sun, 10 Jun 2007, Crispin Cowan wrote:

> Casey Schaufler wrote:
>> --- 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

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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