Disclaimer

The content of this blog is my personal opinion only. Although I am an employee - currently of Imagination Technologies's MIPS group, in the past of other companies such as Intellectual Ventures, Intel, AMD, Motorola, and Gould - I reveal this only so that the reader may account for any possible bias I may have towards my employer's products. The statements I make here in no way represent my employer's position, nor am I authorized to speak on behalf of my employer. In fact, this posting may not even represent my personal opinion, since occasionally I play devil's advocate.

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Thursday, September 06, 2012

Capabilities created on the fly

I'm a fan of capabilities.

But a problem with capabilities systems is creating and maintaining the capabilities.

If somebody has to design the capabilitis, then you end up with a finite list for whatever the creator anticipated you might ask for.  Something like Android's app privileges: "This app requests full Internet access."

Hell, no: I only want it to access particular sites.  And I only want it to send data from certain files. ...

But what OS writer wants to code all of that up?

--

How about creating capabilities on the fly.

E.g. create capabilities automatically for all syscalls:

"open( function_or_regexp_to_be_applied_filename, function_to_be_applied_to_permissions)"

like

open
only files under such and such a directory
    owned by such and such a user
    after a scan has ben done on them
read-only


"* ... "

any syscall
with a filename argument that meets certain criteria ...
...



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Since syscalls are rather low level, might apply this to any function call or library.

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In general, want functions to be applied before the call, given the call, arguments.  Possibly caller...  (like "No socket opens from the user interaction facility".

Possibly evaluate on the raw call and args.
Better yet if can be evaluated on the canonicalized data that might be recorded in a log file - e.g. where filenames are made absolute, etc.

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This doesn't eliminate the need to design coherent capabilties systems.

But it does mean that you can fairly create iron-clad invariants, such as only files in a particular place are accessed.




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