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Comment by Analemma_ | Hacker News Reader

I don't think this is true. There was already a blackbox analysis of the SE presented at Blackhat and it concluded it seemed pretty secure: https://news.ycombinator.com/item?id=12231758. They specifically mentioned that there wasn't much code in the SE and so the attack surface was pretty small.

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From the Blackhat presentation:

  Runs its own operating system (SEPOS)
  * Includes its own kernel, drivers, services, and
  applications

OTOH, the enclave is a separate processor entirely, unlike (AFAIU) on Android phones. And applications on the processor run outside the TrustZone, so there are really two enclaves: the outer enclave (the security processor), and the inner enclave (the TrustZone on the security processor). The hardware restrictions on IPC (both ways between the main processor and security processor) also seems really nice. And they're using an L4 derivative for the OS.

So, yes, it seems like a better design. Very nice. Still, I hold to my criticism. At the end of the day, complexity is anathema to security. If you build an OS, people will use and abuse it. The presentation also mentions that most Apple drivers have now moved partly to the enclave,

  * Most drivers have a corresponding app in the
  SEP

There's a tension there, like with DRM. The more people rely on the enclave to run sophisticated software, the more pressure there'll be to add more features and more complexity. Again, Apple may be better positioned than most to push back, but being smart will only get you so far.

Is SEPOS based on an open-source or proprietary derivative of L4?

From slide 60:

  SEPOS
  * Based on Darbat/L4-embedded (ARMv7)
    - Custom modifications by Apple

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