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breakglass/README.md
Michael Stapelberg d3f89caa0a README: use sdx to prevent mistakes
2019-02-07 14:41:23 +01:00

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breakglass

breakglass is a gokrazy package which provides emergency/debugging access to a gokrazy installation.

It breaks the gokrazy model in that it allows you to run payloads implemented in any language (e.g. busybox, implemented in C).

To repeat, breakglass’s whole idea is remote code execution (via SSH/SCP, listening only on private network addresss). Hence, it should usually not be present on your gokrazy installation, but it might be useful for development/debugging. As a safety measure, breakglass will not automatically be started on boot, but needs to explicitly be started via the gokrazy web interface.

Installation

Add the github.com/gokrazy/breakglass package to your gokr-packer command, e.g.:

gokr-packer -overwrite=/dev/sdx \
  github.com/gokrazy/hello \
  github.com/gokrazy/breakglass

On the permanent file system of your gokrazy installation, create a host key and an authorized keys file. Assuming you mounted the permanent file system at /media/sdx4:

sudo ssh-keygen -N '' -t rsa -f /media/sdx4/breakglass.host_key
sudo install -m 600 ~/.ssh/authorized_keys /media/sdx4/breakglass.authorized_keys

Usage

  1. Create a tarball containing your statically linked arm64 binaries and any other files you’ll need.
  2. SCP that tarball to your gokrazy installation, where breakglass will unpack it into a temporary directory.
  3. Execute a binary via SSH.

Here’s an example, assuming you unpacked and statically cross-compiled busybox in /tmp/busybox-1.22.0 and your gokrazy installation runs on host gokrazy:

$ cd /tmp/busybox-1.22.0
$ file busybox
busybox: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked,
for GNU/Linux 3.7.0, BuildID[sha1]=c9e20e9849ed0ca3c2bd058427ac31a27c008efe, stripped
$ ln -s busybox sh
$ tar cf breakglass.tar --dereference sh
$ scp breakglass.tar gokrazy:
$ ssh gokrazy
/tmp/breakglass564067692 # df -h
Filesystem                Size      Used Available Use% Mounted on
/dev/root                60.5M     60.5M         0 100% /
devtmpfs                445.3M         0    445.3M   0% /dev
tmpfs                    50.0M      1.8M     48.2M   4% /tmp
tmpfs                     1.0M      8.0K   1016.0K   1% /etc
/dev/mmcblk0p4           28.2G     44.1M     26.7G   0% /perm