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rabbit_r1_boot_notes

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Documenting the boot process of the Rabbit R1 (MediaTek mt6765, k65v1_64_bsp, Android 13)

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# Rabbit R1 Boot Notes
REing/documenting the boot process of the Rabbit R1 (MediaTek mt6765, k65v1_64_bsp, Android 13 AOSP).

A lot of the information here will be generic to other Android and/or MediaTek devices, but my intention is only to document the specifics of the R1 device.

This is a living document.

# Glossary

There are soooo many acronyms and abbreviations, so here's a big list for reference:

- `ATF`, `TF-A`: [ARM Trusted Firmware](https://community.arm.com/oss-platforms/w/docs/483/trusted-firmware-a)
- `brom`: bootrom
- `BSP`: Board Support Package
- `DA`: Download Agent
- `EL0`, `EL1`, `EL2`, `EL3`: [ARM Exeption Levels](https://developer.arm.com/documentation/102412/0103/Privilege-and-Exception-levels/Exception-levels)
- `GPT`: [GUID Partition Table](https://en.wikipedia.org/wiki/GUID_Partition_Table)
- `GZ`: [GenieZone](https://patchwork.kernel.org/project/linux-mediatek/patch/20230919111210.19615-2-yi-de.wu@mediatek.com/), EL2 Hypervisor (not to be confused with gzip!)
- `IPI`: InterProcessor Interrupt
- `LK`: [Little Kernel](https://github.com/littlekernel/lk)
- `magiskboot`: A component of [Magisk](https://github.com/topjohnwu/Magisk) for repacking android boot images (kernel, ramdisk). Also repackaged as a standalone tool [here](https://github.com/ookiineko/magiskboot_build).
- `MT`, `MTK`: MediaTek
- `mtkclient`: https://github.com/bkerler/mtkclient - "MTK reverse engineering and flash tool"
- `Picachu`: "PI CAlibration and CHaracterization Utility" - "voltage calibration during booting" (What is PI? Power Input?)
- `REE`: Rich Execution Environment
- `SEJ`: "Security Engine with JTAG control". Possibly the same thing as "HACC".
- `SSPM`: "System Security Processor Manager", "Secure System Power Manager" (???)
- `TEE`: Trusted Execution Environment

More terms defined [here](https://github.com/cyrozap/mediatek-lte-baseband-re/blob/master/General-Notes.adoc)

# Boot Stages Overview

Boot starts in `brom`, which is baked into the CPU silicon. The CPU starts in AArch32 mode.

The next stage is the Preloader, which is stored in the `boot0` partition of eMMC (not to be confused with the `boot` GPT partition).

Preloader is loaded into SRAM at offset `0x200f10`, with entrypoint at `0x201000`. The CPU is still in AArch32 mode here.

The Preloader's stack spans from `0x200000` to `0x200c00`.

The Preloader is responsible for showing the initial boot logo, charging animations, etc. (logo image data is stored in the `logo` GPT partition).

The Preloader loads cached DRAM calibration data from the `boot_para` GPT partition.

The Preloader loads the `lk`, `tee` (ATF), and `gz` GPT partitions into DRAM, and verifies their signatures.

~~ATF is loaded at `0x4800_0000`, which is also where its entrypoint is. Once ATF has initialised, it jumps to GZ in EL2 context.~~ (wrong, LK is loaded at 0x4800_0000, it seems that the LK entrypoint is responsible for calling ATF, which in turn calls GZ? Not really sure...)

GZ entrypoint is at `0x2_4780_0000` (which makes little sense, since that's outside of the DRAM range???)

GZ appears to be itself based on LK also ("welcome to lk/MP" in uart logs)

(TODO: move some of this "overview" stuff into the subsections below)

## BROM

The CPU starts in AArch32 mode.

As far as I can tell, the ROM itself is mapped at address 0.

Among the first things it does is:

- The stack is set up to grow downwards from `0x0010_2738`
- Copy data from `0x0001_0618` to `0x0010_2740`-`0x0010_2891` (presumably `.data`)
- Zero the range `0x0010_2894`-`0x0010_2d00` (presumably `.bss`)

## Preloader

The Preloader also starts in AArch32 mode.

The Preloader is responsible for (in no particular order):

- Initialising DRAM
- Showing the boot logo (and associated animations, e.g. charging)
- Loading and jumping to the next stage (I haven't figured out the specifics of this yet, I think it also switches to AArch64)
- Optionally booting into "Fastboot" mode.
- Optionally booting a "DA" image, loaded over USB.

## LK

LK implements [Android Verified Boot 2.0](https://android.googlesource.com/platform/external/avb/+/main/README.md).

# \[In\]secure Boot

(NOTE: This section is very WIP and may contain inaccurate information. I'll try to verify this stuff soon)

Bootloader lock status is stored in the `seccfg` partition. (i.e. whether Preloader will allow custom `boot` images to boot).

The format of this data is "SecCfgV4", and you can see the parsing logic implemented in mtkclient [here](https://github.com/bkerler/mtkclient/blob/a789e6ccb5601e931a4f4a1f2c3f36fe59c29a81/mtkclient/Library/Hardware/seccfg.py#L11-L102) (nb, it's the `hwtype = "V2"` codepath that gets followed).

The configuration data is hashed, and that hash is encrypted using a hardware crypto engine (`sej`). The encrypted hash acts as a signature of sorts.

As a security control this is ineffective for two reasons:

1. IIUC, The "kamakiri" exploit allows for key dumping (since it gains code exec in brom context).
2. IIUC, the DA can be asked nicely to encrypt/decrypt arbitrary data (no exploits needed!!!)

# Physical Memory Map

```
start       - end (inclusive)

0x0000_0000 - 0x????_????: BROM

0x0010_0000 - 0x0011_2000: SRAM
0x0020_0000 - 0x0030_0000: SRAM (1MB)

0x1000_7000 - 0x????_????: WDT

0x1100_2000 - 0x1100_2020: UART
0x1100_3000 - 0x1100_3020: UART

0x4000_0000 - 0x1_4000_0000: DRAM (4GB)
```

# Goals / TODOs

- Write a minimal standalone mtkclient-like program that only does the things I need it to do (status: prototype)
- Port said client to WebUSB (status: prototype)
- Figure out how to boot a patched LK (Done!!!)
- Write "patchfinder" logic for LK offsets. (python prototype, needs porting/integrating into C my code)
- Figure out how to boot a custom `boot` via my patched LK (perhaps push it over USB before jumping to LK, and patch LK not to load from disk?) (DONE!)
- Patch/hook LK to display a custom boot logo, for style points.
- Figure out how to spawn adbd as root (and selinux unrestricted)

# References

- https://juejin.cn/post/6844904089663307790 - Mediatek boot process overview (in Chinese)
- https://research.nccgroup.com/2020/10/15/theres-a-hole-in-your-soc-glitching-the-mediatek-bootrom/
- https://commoncriteriaportal.org/files/epfiles/NSCIB-CC-0486650-ST-v1.91.pdf
- https://gist.github.com/lopestom/ce250f5de64a2764ee85092a2c01939e - android partition name info
- https://github.com/u-boot/u-boot/blob/master/doc/README.mediatek - u-boot mediatek docs
- https://github.com/bkerler/mtkclient/blob/a789e6ccb5601e931a4f4a1f2c3f36fe59c29a81/mtkclient/config/brom_config.py#L974-L1001 - mtkclient chipconfig (useful memory addresses)
- https://github.com/cyrozap/mediatek-lte-baseband-re/blob/master/General-Notes.adoc MTK baseband reversing notes, includes glossary

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