Modifying grub.cfg in CBFS

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Read Canoeboot flashing guides before continuing, and make sure to back up the current flash contents before you consider following this guide.

Before you follow this guide, it is advisable that you have the ability to flash externally, just in case something goes wrong.

Canoeboot’s GRUB payload is much more flexible than most other types of boot payload, but can sometimes require hands-on configuration depending on what you want to do.

Canoeboot’s own GRUB configuration automatically scans for one provided by your distro, and this automation will usually work. Sometimes, you might wish to override it with your own custom menuentry or additional logic in the GRUB config. You can configure GRUB however you like, and this topic is vast so what to actually put in the config will not be covered here.

This guide will simply teach you how to modify the config, but not what to put, whereas the GRUB hardening guide specifically says what to modify; cross reference that page and this page, if you want to follow the GRUB hardening guide. Otherwise, this page contains generic guidance.

Before internal flashing, you must first disable /dev/mem protections. Make sure to re-enable them after you’re finished.

See: Disabling /dev/mem protection

This only applies if you’re following these instructions via internal flashing, from an existing installation. If you’re externally flashing the machine, you can ignore this advice.

Please first install build dependencies.

You need cbfstool from coreboot. For whatever board you have, check which coreboot tree it uses in Canoeboot’s build system, cbmk. For example, let’s say your board is x200_8mb, you would do:

grep tree= config/coreboot/x200_8mb/target.cfg

In this example, the output might be:

tree="default"

This means you should compile cbfstool from the default coreboot tree, like so:

./mk -d coreboot default

This will result in the following binary: elf/cbfstool/default/cbfstool

We won’t assume the path to cbfstool, in the remainder of this guide, so adapt accordingly.

Again, let’s assume the coreboot board is x200_8mb. Check the file config/coreboot/x200_8mb/target.cfg for grubtree - if it’s not set, then the GRUB tree is default. We will assume default:

./mk -b grub default

This will compile GRUB for the given tree. If you need to use any of the GRUB utilities, this command will build them and in this example, they will be available under src/grub/default/.

Compile flashprog like so:

./mk -b flashprog

A binary will appear at elf/flashprog/flashprog.

The coreboot image has its own filesystem, CBFS, and within CBFS is the GRUB binary, and within the GRUB binary is another filesystem called memdisk, where the default GRUB configuration is located.

You can override it by inserting your own GRUB config within CBFS.

This is only useful if you already inserted a GRUB config in CBFS. Otherwise, you can grab it from Canoeboot’s build system, cbmk.

Learn how to externally reprogram these chips and use the -r option in flashprog; alternatively, for internal flash access, look at the main flashing guide.

Those guides show how to dump the flash contents, which you are advised to do.

We’ll assume that your GRUB tree is default, so the file config/grub/default/config/payload is your GRUB config; this will be the same as what you have in memdisk. Make a copy of this file, for modification.

Modify that file, or the one you extracted if you already inserted a custom one before, and you will re-insert it when you’re done.

Before reading the next section, please note: if you only have the fallback GRUB config in memdisk, and no configs in CBFS, you can test the modified version by inserting it as grubtest.cfg, instead of grub.cfg:

cbfstool canoeboot.rom add -f grubtest.cfg -n grubtest.cfg -t raw

Canoeboot will not automatically load it, but it will be available from the default GRUB menu. This can be useful for test purposes, hence the name.

If you already have a grub.cfg in cbfstool, you can extract and modify that one, e.g.:

cbfstool canoeboot.rom extract -n grub.cfg -f grub.cfg

Now remove it:

cbfstool canoeboot.rom remove -n grub.cfg

It’s important that you re-add grub.cfg before flashing (or just add it, if it was never there in the first place):

cbfstool canoeboot.rom add -f grub.cfg -n grub.cfg -t raw

**If you flash the Canoeboot image without a grub.cfg in CBFS, it will default back to the one in GRUB memdisk.

AGAIN:

If you only want to test your changes first, insert it first as grubtest.cfg and ensure that no grub.cfg exists in flash. This will let you access the test config from the default menu, before deciding whether to make it the main config, as grub.cfg, overriding the one in GRUB memdisk.

Check the Canoeboot flashing guide which says how to flash the new image.

Markdown file for this page: https://canoeboot.org/docs/linux/grub_cbfs.md

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