heads-wiki

Documentation for the Heads firmware project

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Boot config files

A user has the option to make persistent modifications to the non-Qubes boot process by creating one or more of the following files:

file description
kexec_menu.txt contains multiple options for parameters to the kexec command
kexec_hashes.txt a sha256sum file from within the respective boot directory
kexec_iso_add.txt a sh variable to override the standard ISO kernel argument additions
kexec_iso_remove.txt a sh variable to override the standard ISO kernel argument removals
kexec_default.$N.txt specifies the default kexec parameters corresponding to the Nth menu option
kexec_default_hashes.txt a sha256sum file for the default entry kexec file parameters
kexec_rollback.txt a sha256sum of the TPM counter contents in the tmp directory
kexec_key_devices.txt contains a list of “device uuid” combos for all LUKS devices to unlock
kexec_key_lvm.txt contains the name of an LVM group to activate on boot

These can be placed in any of the following locations:

location description
/boot/ used during internal HD boot
/media/ used during standard USB boot
/media/kexec_iso/$ISO_FILENAME/ used during USB boot from a particular ISO file

These files are only used if there is an appropriate signature for them in kexec.sig covering all kexec*.txt in that location. This can be generated by running kexec-sign-config -p /boot/, etc. These files are copied by kexec-check-config to /tmp/kexec/ only there’s a valid signature. From there the boot routines reference only the configs in /tmp/kexec.

If there is no persistent kexec_menu.txt, the boot directory will be searched for grub/syslinux-like configurations and it will be generated on-the-fly (for any of the HD/USB/USB-ISO locations). Creating a persistent kexec_menu.txt can be useful to limit the options displayed or to make custom persistent alterations to xen or kernel params.

kexec_menu.txt has a simple layout with a single line per boot option:

description 1|elf|kernel /vmlinuz... |initrd /initramfs... |append ...
description 2|multiboot|kernel ... |module ... |module ...
description 3|xen|kernel /xen... |module /vmlinuz... | module /initramfs...

This is a sample kexec_menu.txt covering the expected options (derived from grub.cfg):

Ubuntu|elf|kernel /vmlinuz-4.8.0-58-generic|initrd /initrd.img-4.8.0-58-generic|append root=/dev/mapper/ubuntu--vg-root ro quiet splash crashkernel=384M-:128M crashkernel=384M-:128M
Memory test (memtest86+, serial console 115200)|elf|kernel /memtest86+.bin|append console=ttyS0,115200n8
Qubes, with Xen hypervisor|multiboot|kernel /xen-4.6.5.gz placeholder |module /vmlinuz-4.4.67-13.pvops.qubes.x86_64 placeholder root=/dev/mapper/luks-UUID ro rd.qubes.hide_all_usb|module /initramfs-4.4.67-13.pvops.qubes.x86_64.img

If there is a persistent kexec_hashes.txt, a non-default boot will fail when the file hashes don’t match the expected values. By default, no such checks are made.

When booting from an ISO file on a USB drive, it must be signed by a valid key in the Heads ROM and the boot process will fail if invalid. The kexec_iso_add.txt and kexec_iso_remove.txt are useful to inject the appropriate kernel arguments to allow it to load properly. ISOs for Debian require that kexec_iso_add.txt contains to load properly:

findiso=${ISO_PATH}

Take a look at http://mbusb.aguslr.com/howto.html for more variations on the distro-specific ISO mounting command lines requirements. By default Heads uses two variants of this when booting from ISO where a kexec_iso_add.txt is not specified:

fromiso=/dev/disk/by-uuid/$DEV_UUID/$ISO_PATH iso-scan/filename=/${ISO_PATH}

Note that currently, any multiboot entry is interpreted as a Xen-variant and kexec-boot overrides the arguments to the multiboot kernel with custom arguments. A user can manually specify multiboot entries to override the default behavior by creating a custom kexec_menu.txt.

If a user wishes to require that file hashes be checked for a succesful non-recovery boot, they may set the CONFIG_BOOT_REQ_HASH=y in their respective Heads config file.

As as convenience mechanism, a user may select a boot option to always be used in the future, assuming that the boot parameters and file hashes have not changed. This can be done by running kexec-save-default manually or directly from the boot menu. This works for any boot location (HD/USB/USB ISO) but does modify the respective /boot/ or /media/ filesystems. An entry index is maintained so that if the options are being derived from the live grub.cfg (i.e. no persistent kexec_menu.txt) and when there is a change to the underlying grub parameters, the boot will fail and require the user to resign/revalidate the settings. This is useful to detect changes to the primary kernel/initramfs (for example in the Qubes case when the primary entry is first).

If a user wishes to require that a TPM counter be set for rollback prevention, they may set the CONFIG_BOOT_REQ_ROLLBACK=y in their respective Heads config file. When this is true, standard boot will only succeed if:

1) Booting from an verified ISO 2) Booting from a mount point that has a valid kexec_rollback.txt in its parameter directory

The simplest way to achieve this is to set a default boot option as this updates the rollback counter by default.