TPM v1.2 has a limitation in that only a single monotonic counter can be
incremented between reboots [1]. So in the event we are using HOTP
monotonic counters, we need to reference those for the Heads rollback
counter when we update file signatures in /boot, otherwise the increment
stage at kexec-sign-config will fail since at each boot, the HOTP
monotonic counter has already been incremented.
[1] https://projects.csail.mit.edu/tc/tpmj/UsersGuide.html#inccounter
The Librem Key is a custom device USB-based security token Nitrokey is
producing for Purism and among other things it has custom firmware
created for use with Heads. In particular, when a board is configured
with CONFIG_LIBREMKEY, this custom firmware allows Heads to use the
sealed TOTP secret to also send an HOTP authentication to the Librem
Key. If the HOTP code is successful, the Librem Key will blink a green
LED, if unsuccessful it will blink red, thereby informing the user that
Heads has been tampered with without requiring them to use a phone to
validate the TOTP secret.
Heads will still use and show the TOTP secret, in case the user wants to
validate both codes (in case the Librem Key was lost or is no longer
trusted). It will also show the result of the HOTP verification (but not
the code itself), even though the user should trust only what the Librem
Key displays, so the user can confirm that both the device and Heads are
in sync. If HOTP is enabled, Heads will maintain a new TPM counter
separate from the Heads TPM counter that will increment each time HOTP
codes are checked.
This change also modifies the routines that update TOTP so that if
the Librem Key executables are present it will also update HOTP codes
and synchronize them with a Librem Key.
In addition to being able to flash a ROM from the GUI, it would also be
useful for a user to be able to add a GPG key to their keyring using the
flashing tool. This change adds the ability for a user to edit both a
ROM located on a USB key and also edit the running BIOS by using
flashrom to make a local copy of the running BIOS, edit it, then reflash
it. This also supports the upcoming delete feature in CBFS for
circumstances where keyring files already exist within CBFS.
If we want to modify a running BIOS we will need the ability to pull
down the current BIOS, modify it, and then reflash. This change adds a
read option to flash.sh and pulls down three versions of the BIOS and
only exists successfully if all three match.
To keep the flash logic simpler the GUI logic has been split into a
flash-gui.sh program so flash.sh behaves closer to the original flashrom
scripts it was based from. I've also removed the previous flashrom
scripts and incorporated their options into flash.sh. Finally I set
CONFIG_BOARD via the Makefile instead of setting a duplicate option in
each board's config.
Based on the conversation for PR #406, we decided to go with a more
generic script for general-purpose flashing instead of having individual
(and therefore very similar) flash scripts for each board type. This
script currently handles flashrom on Librem and X230 board types and
introduces a new CONFIG_BOARD option that sets specific flashrom
arguments based on the board.
It also adds support to gui-init to call this flash script.
Currently the TPM counter label is hard-coded. By changing it to a
variable in this function we can reuse all of the TPM counter functions
to create other monotonic counters in the TPM (if the hardware supports
it) with custom labels.
Wrapping text to 80 characters works but due to font size and padding
the maximum 80 character lines start to get truncated. Extending the
window to 90 characters will resolve this.
While the whiptail program wraps text appropriately based on column
size, the fbwhiptail program doesn't, leading to text that scrolls off
the window where it can no longer be read. This change wraps the longer
text output so it all fits.
Since fbwhiptail allows us to customize the background colors, we should
colorize warnings and error messages to provide a user with an
additional subtle cue that there might be a problem. I have added two
additional configuration options:
CONFIG_WARNING_BG_COLOR
CONFIG_ERROR_BG_COLOR
and in the librem13v2.config file you can see an example for how to set
them to be yellow and red gradients, respectively. I've also updated the
main two scripts that use whiptail to include those background colors.
If you decide to use regular whiptail, just don't set these config
options and it should behave as expected.
When a user gets confirmation of their boot menu choice, that's largely
to give them the option of making their boot choice the default. In the
case of "force mode" there's no reason for the user to be presented with
that dialog so this change skips right ahead to the boot once they have
In the event a user does pick the insecure "force" boot option that
bypasses checksum and signing checks in Heads, it would be nice to
provide a clear visual warning during the boot process that they are in
this state. This change will add a kernel argument that changes the boot
console background to be red and removes any boot splash that might
obscure it, in the event the user picks the insecure boot mode.
Since a user should only boot into this mode during emergencies, having
it be apparent that it's an unsafe mode helps ensure the user doesn't
pick this boot option needlessly.
Currently when the boot entries change, kexec-select-boot dies. Given
the normal loop is set up to catch this event and display a regular boot
menu at the next iteration of the loop, instead of dying it would be
better to just warn and then return from that function back into the
main loop. In addition to that I added a GUI menu for the same warning
when in GUI mode.
Part of the Heads workflow involves handling legitimate changes to /boot
as part of the package manager. This is a challenging workflow to handle
as package managers on many systems work in a completely unattended way
(and some even reboot first, apply updates, and then reboot again).
We need to be able to detect changes that are potentially caused by a
package manager so to do that I've set up a trigger within the OS
(currently just for Debian) that runs both before and after package
updates. It verifies the signatures in /boot and if they fail before
package updates it creates a log file in
/boot/kexec_package_trigger_pre.txt. If they fail after package updates
run /boot/kexec_package_trigger_post.txt is created. These files contain
the following fields:
CHANGED_FILES: A list of files in /boot that failed the sha256sum check
UPDATE_INITRAMFS_PACKAGE: An (optional) list of packages known to
trigger initramfs changes
Following those fields is a list of log output from the last package
manager run which contains its own formatted fields (I'm pulling from
/var/lib/dpkg/info).
When a user selects a boot option, gui-init first verifies the
checksums just to catch errors before calling kexec-select-boot. If
there are any errors it looks for these package logs and if they exist,
it displays appropriate warnings. If the files are absent it displays a
more generic warning. The user is also given an opportunity to re-sign
the /boot hashes.
