Comment by londons_explore
2 years ago
What are the chances this MMIO register could have been discovered by brute force probing every register address?
Mere differences in timing could have indicated the address was a valid address, and then the hash could perhaps have been brute forced too since it is effectively a 20 bit hash.
It looks like the registers could have been identified fairly easily via brute force. They're physically close to documented GPU registers, and accessing them triggers a GPU panic, which is how the researchers attributed them to the GPU component. The attackers could have used that same test to identify the existence of the registers.
The part that's less easily explained is how they were able to reconstruct a custom sbox table to execute the debug code. That's where the "insider threat" insinuations are strongest, but personally I'm not convinced that it precludes any number of other plausible explanations. For example, the attackers could have extracted the sbox from: older firmwares, OTA update patches, pre-release development devices (probably purchasable on ebay at some points), iOS beta releases, or a bunch of other leaky vectors.
The researcher basically says "I couldn't find this sbox table in any other binary where I looked for it." Well, that's not necessarily surprising since it appears to be Apple specific and thus there are a limited number of binaries where it might have appeared. And as the researcher notes, this includes now unpublished binaries that might have been mistakenly released. It's totally plausible that the attackers got lucky at some point while they were systematically sniffing for this sort of leak, and that the researcher is unlikely to have the same luck any time soon.
Looking at that sbox implementation, I can't believe it was implemented as a lookup table in the hardware of the chip - there must be some condensed Boolean expression that gives the same result.
The fact the attackers didn't know that Boolean expression suggests they reverse engineered it rather than had documentation.