Comment by mrandish

> What kind of hardware interface does arbitrary DMA protected by a secret but weak hash function?

I agree. This appears to likely be an intentional backdoor injected at the hardware level during design. At such a low-level I think it could have been accomplished with only a handful of employees in on it. There would have been no need to subvert Apple from the top down with large numbers of people at many levels being privy.

In early silicon there can be a bunch of registers and functions implemented for testing which are later pulled out. Except maybe one set of registers doesn't get pulled but instead a door knock is added with a weak hash function, making the registers invisible to testers and fuzzing.

It seems a little too convenient that the door knock hash was weak. After all, strong hash functions aren't unknown or hard. The reason it had to be a weak hash function was to create "plausible deniability". If it was a strong hash then once any exploitation was discovered there would be no denying the vuln was intentionally placed. If it really was just a test DMA function that someone supposedly 'forgot' to remove before production silicon, I can't think of a reason to have it behind any kind of door knock in the first place.

I read that it was patched by adding these addresses to the "access denied" list. While I don't know anything about Apple security, I'm stunned that any such low-level access list isn't 'opt-in' instead of 'opt-out'. If it was 'opt-in' it seems like any such 'undocumented' register addresses would by denied by default. And if they were on the 'opt-in' list, yet remained undocumented, then it would be obvious to anyone looking at the security docs that something was amiss.