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Script for flashing coreboot on MacBooks without using external SPI programmer
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Evgeny Zinoviev
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README.md

mmga: Make MacBook Great Again

mmga is a script to help flashing coreboot on some MacBook Air and Pro models without using external SPI programmer.

Supported models

As of time of writing, following models are supported in coreboot. Other models might be supported in future.

  • MacBook Air 4,2 (13'' Mid 2011) (macbookair4_2)
  • MacBook Air 5,2 (13'' Mid 2012) (macbookair5_2)
  • MacBook Pro 8,1 (13'' Early 2011) (macbookpro8_1)
  • MacBook Pro 10,1 (15'' Mid 2012 Retina) (macbookpro10_1)

System requirements

  • Recent Linux booted with iomem=relaxed kernel parameter (needed for internal flashing to work);
  • Build dependencies. Here's a list for Ubuntu 16.04:
    # apt install bison build-essential curl flex git gnat libncurses5-dev m4 zlib1g-dev make libpci-dev libusb-1.0-0-dev
    
    On other distros package names might differ. Be sure to install gnat prior to building coreboot toolchain.

Building flashrom

First of all, grab recent flashrom source tree and build it:

$ git clone https://review.coreboot.org/flashrom.git && cd flashrom
$ make

Optionally you can install it to /usr/local/sbin:

$ sudo make install

How it works

The firmware is stored on SPI chip. On Intel platforms it consists of various regions: fd (Flash Descriptor), me (Intel ME), bios (the BIOS, or, in case of MacBooks, EFI), and some other (such as gbe for Gigabit Ethernet). The most important region in context of this story is FD, the Intel Flash Descriptor.

The Intel Flash Descriptor is a data structure stored on the flash chip; it contains information such as space allocated for each region of the flash image (also called a layout), read-write permissions for each region and many more. The Flash Descriptor is located at the first 4K of the SPI chip (0x0000-0x0fff).

Here's flash chip layout used in MacBook Air 5,2 (which has 8M flash chip), extracted from original firmware dump with ifdtool:

00000000:00000fff fd
00190000:007fffff bios
00001000:0018ffff me

As you can see,

  • the first region (0x0000-0x0fff) is used for a Flash Descriptor (as it always is);
  • right next to it (0x1000-0x18ffff) Intel ME firmware is stored;
  • and the last and largest region (0x190000-0x7fffff) is the BIOS, or Apple's EFI in our case.

Normally, the FD should be read-only, but this is not the case with MacBooks. Apparently, Apple's "Think Different" (TM) thing applies to firmware security as well.

You can check access permissions on your MacBook by running flashrom -p internal (if it doesn't work, make sure you have booted with iomem=relaxed and/or use -p internal:laptop=force_I_want_a_brick instead). This is what it shows on MacBook Air 5,2 with latest Mojave firmware updates:

# flashrom -p internal
flashrom v1.1-rc1-3-g4ca575d on Linux 4.9.0-9-amd64 (x86_64)
flashrom is free software, get the source code at https://flashrom.org

Using clock_gettime for delay loops (clk_id: 1, resolution: 1ns).
No DMI table found.
Found chipset "Intel QS77".
Enabling flash write... SPI Configuration is locked down.
PR0: Warning: 0x00190000-0x0066ffff is read-only.
PR1: Warning: 0x00692000-0x01ffffff is read-only.
At least some flash regions are write protected. For write operations,
you should use a flash layout and include only writable regions. See
manpage for more details.
OK.
Found Micron/Numonyx/ST flash chip "N25Q064..3E" (8192 kB, SPI) mapped at physical address 0x00000000ff800000.
No operations were specified.

As you can see, only the bios region is read-only, and not even whole bios region, because 0x670000-0x681fff is writable for some reason:

PR0: Warning: 0x00190000-0x0066ffff is read-only.
PR1: Warning: 0x00692000-0x01ffffff is read-only.

It's interesting that this behavior is reproducible only after cold boot. If you suspend to S3, resume and run flashrom again, fd will be read-only:

PR0: Warning: 0x00000000-0x00000fff is read-only.
PR1: Warning: 0x00190000-0x0066ffff is read-only.
PR2: Warning: 0x00692000-0x01ffffff is read-only.

Looks like a bug in Apple's firmware. Obviously it should always be read-only.

Anyway, that means that after cold boot fd and me regions are writable, and that gives us around 1.5M of writable space. Since we can rewrite FD, we can write a new FD with custom layout. So the idea is to repartition the flash chip and flash new bios to a writable space.

Let's write a new layout (I decided to use 0x00000-0xfffff region for convenience):

00000000:00000fff fd
00001000:00020fff me
00021000:000fffff bios
00100000:007fffff pd

In this layout, we allocate 128K for me and 892K for bios. To fit the original 1.5M ME image into the 128K region, it has to be truncated with me_cleaner with -t and -r arguments, the size of resulting image is ~92K. We also have to allocate the remaining 0x100000-0x7fffff region for something to be able to address and flash it in future, otherwise flashrom will give us a "Transaction error". So we just mark it as pd, which stands for "Platform Data".

After the new layout is ready, we build small coreboot ROM that fits into the allocated 892K bios region. We do that, then flash fd (0x0000-0x0fff), me (0x1000-0x20fff) and bios (0x21000-0xfffff) according to the new layout. On the next cold boot, coreboot will be loaded from the 0x21000-0xfffff region, and old firmware, which still resides in 0x190000-0x7fffff, will be ignored. This is stage1.

After we boot into just-flashed coreboot, we're able to flash the whole 8M chip, because it's not write-protected anymore. We repartition the chip again, and the new layout looks like this:

00000000:00000fff fd
00001000:00020fff me
00021000:007fffff bios

It's almost the same, except that bios fills all the remaining space. Then we build coreboot again, flash fd, me and bios and shut down again. On the next cold boot we will have completely corebooted MacBook. This is stage2.

Usage instructions

The mmga script automates steps described above and does all dirty work.

Usage:
./mmga <options> ACTION
Options:
-h, --help: show help
stage1 actions:
          dump: dump flash contents
         fetch: fetch board tree from Gerrit (if needed)
prepare-stage1: patch IFD, neutralize and truncate ME
 config-stage1: make coreboot config (for manual use)
  build-stage1: make config and build ROM (for auto use)
  flash-stage1: flash ROM ($COREBOOT_PATH/build/coreboot.rom)
stage2 actions:
prepare-stage2: patch IFD (if needed)
 config-stage2: make coreboot config (for manual use)
  build-stage2: make config and build ROM (for auto use)
  flash-stage2: flash ROM ($COREBOOT_PATH/build/coreboot.rom)
other actions:
     flash-oem: flash OEM firmware back

Configuration

Before you start, you have to update variables the config.inc file:

  • PAYLOAD: which payload to use, supported values are grub and seabios
  • MODEL: put your macbook model here, example: macbookair5_2
  • GRUB_CFG_PATH: only if you use grub payload; if empty, default grub.cfg will be used
  • COREBOOT_PATH: path to cloned coreboot repository (see below)
  • FLASHROM: path to flashrom binary
  • FLASHROM_ARGS: in case if flashrom detects multiple flash chips, put "-c CHIP_MODEL" here
  • STAGE2_USE_FULL_ME: if you want to use original Intel ME image in the final ROM for some reason, set to 1

stage1

Get coreboot:

$ git clone --recurse-submodules https://review.coreboot.org/coreboot.git && cd coreboot

Build coreboot toolchain. You must have gnat compiler installed as it's required for graphics initialization to work (libgfxinit is written in Ada):

$ make crossgcc-i386 CPUS=$(nproc)
$ make iasl

Dump the flash chip contents:

./mmga dump

Create patched FD and neutralize ME:

./mmga prepare-stage1

If your board's port hasn't been merged to coreboot master yet or you don't know, run:

./mmga fetch

Create coreboot config and build the ROM:

./mmga build-stage1

(If you're experienced coreboot user or developer, you may want to configure and build coreboot yourself. In that case, run config-stage1 instead of build-stage1. It will create config that you can then copy to $COREBOOT_PATH, make your changes and build. Please be aware that build-stage1 applies SeaBIOS patch and you will have to apply it manually.)

Flash it:

./mmga flash-stage1

If it's done and you didn't see any errors, you have to shutdown the laptop. It's important: DO NOT REBOOT, shut it down. If you reboot, old FD will still be used and you will boot the Apple's firmware, because it was left untouched in the 0x190000-0x7fffff region. But since you partly replaced the old me region, it may lead to undefined behaviour. So again, do not reboot, shut it down.

stage2

Create patched FD for the next flash:

./mmga prepare-stage2

Create new coreboot config and build the ROM (for experienced users, config-stage2 is also available):

./mmga build-stage2

Flash it:

./mmga flash-stage2

If it's done and you again didn't see any errors, you have to shutdown the laptop again. DO NOT REBOOT, shut it down. It's even more important now: if you reboot, old FD will be used, the one that describes bios region as 0x21000-0xfffff. And since you just flashed bios to 0x21000-0x7fffff, this 0x21000-0xfffff will most likely just contain FFs, so the laptop won't boot and will look like a brick. In that case you will need to press and hold power button for ~10 seconds to hard reset. To avoid all that, just do not reboot, shut it down.

Misc

The script was tested on MacBook Air 5,2 and MacBook Pro 10,1. If you have successfully corebooted your macbook with it and it worked, please let me know. If you have any problems, contact me via GitHub issues or by email (see the copyright header in the script).

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