Ultimately, yes, this seems to be a memory controller register that should not be exposed to ring 0 or above.

Ring -1 needs DRAM, so it tells the memory controller to give it some blocks. The memory controller hands back a “physical” address, and promises not to let anything but ring -1 access that address.

The exploit takes advantage of that control register to remap the same DRAM blocks to a different physical address. Since the memory controller only promised to protect the physical address it handed back, that protection is bypassed when using the new address.

There are several theoretical ways to mitigate this exploit, but it remains to be seen if the system is sufficiently field-upgradeable to defend.