kik-website/server

The shell server

The website's terminal connects to a shell server, whose root user logs in at wss://web.nowhere.wirs.ing and gets a shell on the box.

The security model

The box is an isolated, disposable VPS at a random hoster. We assume that it can be compromised and that this is acceptable: the worst case is that the gimmick on the website stops working. Nothing else is on the box. The website and the websocket bridge that carries the connection run on other servers, so a compromised shell server cannot get at them.

The root user on the box is a normal Windows account without administrator rights. It may do a lot of things there, including things we did not plan for. We do not try to contain it and we do not try to undo what it does. We only keep the basic user journey working.

What we keep working

Anything that breaks the following user journey is not acceptable:

• The user can connect with the SSH key as root.
• The prompt: root@khaos-im-kiez:~#, which ~/.zsh/.zshrc sets.
• The home directory the user ends up in: /home/root. They cannot change where it is, and they cannot delete it.
• cat README, cat README.en, cat CODE_OF_CONDUCT and cat CODE_OF_CONDUCT.en always work. The four files are always there and always regular text files.

So in summary running ssh root@nowhere.wirs.ing cat README must always succeed and display the contents of the file.

Anything thats not specified above (like the content of README for example) is NOT part of the user journey.

What we do not keep working

As long as the user story above does not break, we do not guarantee the functionality of anything else on the server. However the cleanup task should still try to best-effort restore everything to a working condition.

Packages

The root user owns the whole MSYS2 installation: they can install packages with pacman, and change and delete everything in it. Protected is only what the sshd trusts, because the sshd runs as SYSTEM and a file it trusts that the user wrote would hand them the box:

• the sshd, the login shell zsh and cat: a second copy in C:\msys2\opt\kik, which the server runs from and whose zsh is the login shell, so the journey keeps working no matter what the user does to their own /usr/bin
• the runtime /usr/bin/msys-2.0.dll
• /etc/passwd and /etc/group, which decide which Windows account a login becomes - a changed one could make the website's key log in as the Administrator
• /etc/fstab, /etc/fstab.d and /etc/nsswitch.conf, which decide which files those paths are and how accounts are looked up
• /etc/ssh, the configuration and the host keys of the server

The user may read these at most. Their folders - C:\msys2, etc, usr, usr\bin and opt - belong to the administrators and cannot be renamed, or the user could put a folder of their own in their place. The user has modify rights on the installation, not full control: full control would also let them delete files whose own permissions forbid it. That is how one visitor deleted the locked /etc/passwd along with the whole installation.

The copy has to be the same MSYS2 installation, not a second one: C:\msys2\opt\kik is two levels below C:\msys2, so its runtime resolves the same root and the login shell and the user's own tools share one cygwin runtime. A separate installation does not work - two runtimes in one process tree corrupt each other's file descriptors and pacman cannot even lock its database. That is also why the runtime itself is not copied but locked where it is: a program and the shell that starts it have to use the same runtime DLL. The price is that the user cannot update msys2-runtime; a deployment brings a new one.

All protected files come from a pristine installation in C:\ProgramData\kik-restore\msys2, built by the installer from the official MSYS2 base archive, whose hash is pinned in the installer, and the packages of the site. Neither the tasks nor the installer ever run or trust anything from C:\msys2: a program the user replaced would run as SYSTEM. For the same reason the Administrator's sessions only search the folders of Windows for programs - they would otherwise inherit the server's path, which leads into /usr/bin. When the user wipes the installation - when pacman is gone - the cleanup job copies everything that is missing back from the pristine one, so the next visitor finds a working system again.

The cleanup tasks

Three scheduled tasks run as SYSTEM, all at startup, and the user cannot stop any of them:

• KiK sshd runs the sshd from C:\msys2\opt\kik. It replaces the installation's own msys2-sshd service, which is disabled.
• KiK restore shell files runs every 30 minutes and puts the protected files back from the pristine installation, re-applies the permissions, and puts the home directory, the shell configuration, the website's key and the four texts back. Its second job is to best-effort revert other mockery and bring a fresh server to the expected state, which includes copying back an installation the user wiped.
• KiK watch the server runs every two minutes and starts the server when nothing listens on port 22. The task of the server only starts it again when it failed, never when it was stopped, so a cleanup job that is killed while it has the server stopped would otherwise take the site down with it.

The cleanup job only restarts the server when it has to: when it does not listen at all, when it is older than its configuration, or when a deployment brought a new protected copy or runtime - those cannot be replaced while the server holds them open. A working server is left alone, because every start can fail, and failed starts have kept the site down for hours. The job also never restarts the server while somebody is connected: a session that is open keeps the port, so a server that is started then could not bind it.

The server is only down for the short part of the job that needs it down: the protected copy cannot be replaced while the server holds it open. Everything else - the home directory, the four texts, the permissions of the installation - is put back while the server runs, because on a slow disk that takes minutes and the site should have a shell the whole time.

The task that runs the server ignores a start while it thinks it is still running, so a start that is dropped leaves the site without a shell. The cleanup job therefore tries three times, killing the processes of the task in between, before it gives up.

The deployment does not restart the server. It runs in a session, and that session keeps the port, so a server that starts during a deployment cannot bind it. The new configuration and the restored programs therefore take effect at the next run of the cleanup job, which happens within half an hour.

The server logs to C:\ProgramData\kik-restore\sshd.log. The log is appended to, not overwritten, because it is the only trace of what the server did: a server that is restarted would otherwise take the evidence of an outage with it. It is rotated when it grows past a hundred megabytes, which takes years. The event logs of windows are made bigger as well, the security log to a gigabyte: the scanners of the remote desktop alone fill a hundred megabytes of it in a week.

A start of the server that fails leaves nothing in that log. So windows records the history of the scheduled tasks and the start and end of every process, and when the server does not come up, the cleanup job writes the processes and the state of the server's task to C:\ProgramData\kik-restore\failed-starts.log.

Only keys may log in over SSH. The box stands in the open internet and both of its accounts have a password.

The files that have to stay belong to the administrators, so the user cannot delete them, and they cannot change the shell configuration or the key the website logs in with either. The shell reads its configuration from ~/.zsh, so the startup files the user could write in the home directory are ignored.

Installing the cleanup on the server

To deploy the cleanup task an the server, run:

server/scripts/deploy.sh

The script logs in as Administrator - whose shell is PowerShell, which nothing in the installation can replace - copies the scripts and the resources to the Administrator's home directory and runs the installer there.

When SSH is down, server/scripts/rdp.sh opens a remote desktop session as Administrator on a virtual display, with a local directory shared into it. The password goes in .password.txt in the root of the repository.