2858 to 1888 Kernel 'Downgrade' Confirmed, not working with 4552+
Date: Tuesday, January 23 @ 13:00:25 UTC
Topic: Homebrew
Robinsod posted an update about his hack to downgrade a 2858 (or lower) kernel back to the 1888 'Base Kernel'. By being able to boot the 'Kiosk disc' from recordable media he proofs his 360 is no longer running the 2858 kernel (which blacklisted these XEXs) and boots without applying the kernel patches that have nulled headers:
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Confirmed, downgrading allows the Kiosk disk to run again!
So I have now proved that kernel 2.0.2858 (won't boot the kiosk disk) can be downgraded to 2.0.1888 and that WILL boot the kiosk disk (or bits of it) from CD-R. This was proved with a mobo for which the DVD key is NOT known.
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However, with newer kernels (4552 and after) it seems to be (currently) impossible to downgrade back to 1888. From Robinsod and Speedy22:
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Until the 4552 update that is exactly what I was doing, erasing/corrupting part of the patch and rebooting. The 360 fell back to the previous patch or, if no patch has been applied, the Base Kernel.
After the 4552 the 360 WILL NOT BOOT unless the 4552 patch is present in flash.
More than likely MS has blown an efuse or two with the 4552 update. The efuses are located in the CPU. Anyone interested in learning more should download my 360 CPU datasheet V1.5 from the beginning of March 2005.
The Hypervisor only needs to test a flag in the processor (blown eFuse) against a flag in the patch to make a boot/no boot decision, there could be any number of these flags but at a rate of 1 eFuse / year then 32 would be more than ample. There's no need to actually modify the hypervisor code at all.
The efuses act like standard memory and probably contain the HID register data as well. I would guess there are 1K-2K worth of efuses.
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Note: Right now there's no use (or easy way) for end-users to downgrade their kernel ... but it's very interesting research and might be useful in the future or for further research.
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