727f8f6e9c
Every outage since 28 Oct began at the cleanup job's routine restart of a working server: the start failed and kept failing until a reboot. The job now leaves a working server alone and restarts it only when it does not listen, is older than its configuration, or holds open a program or the runtime that has to be repaired. A failed start left no trace anywhere, so windows now records the history of the scheduled tasks and the start and end of every process, and the job writes the processes and the state of the server's task to failed-starts.log when the server does not come up. The security log grows to a gigabyte, because the scanners of the remote desktop fill a hundred megabytes in a week. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
114 lines
7.3 KiB
Markdown
114 lines
7.3 KiB
Markdown
# The shell server
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The website's terminal connects to a shell server, whose `root` user logs in at `wss://web.nowhere.wirs.ing` and
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gets a shell on the box.
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## The security model
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The box is an isolated, disposable VPS at a random hoster. We assume that it can be compromised and that this is
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acceptable: the worst case is that the gimmick on the website stops working. Nothing else is on the box. The
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website and the websocket bridge that carries the connection run on other servers, so a compromised shell server
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cannot get at them.
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The `root` user on the box is a normal Windows account without administrator rights. It may do a lot of things
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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.
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We only keep the basic user journey working.
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## What we keep working
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Anything that breaks the following user journey is not acceptable:
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- The user can connect with the SSH key as `root`.
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- The prompt: `root@khaos-im-kiez:~#`, which `~/.zsh/.zshrc` sets.
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- The home directory the user ends up in: `/home/root`. They cannot change where it is, and they cannot delete it.
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- `cat README`, `cat README.en`, `cat CODE_OF_CONDUCT` and `cat CODE_OF_CONDUCT.en` always work. The four files
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are always there and always regular text files.
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So in summary running `ssh root@nowhere.wirs.ing cat README` must always succeed and display the contents of the file.
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Anything thats not specified above (like the content of README for example) is NOT part of the user journey.
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## What we do not keep working
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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.
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## Packages
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The root user owns the whole MSYS2 installation and can install packages with pacman. That means they can also
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replace or delete the very programs the login needs. So the sshd, the login shell `zsh` and `cat` are kept as a
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second, protected copy in `C:\msys2\opt\kik`, which only SYSTEM and the administrators may write. The sshd runs as
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SYSTEM from that copy and the login shell is that copy's `zsh`, with `/opt/kik` first on the path, so the journey
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keeps working no matter what the user does to their own `/usr/bin`. Replacing `/usr/bin/sshd` cannot touch the
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running server, so the key on the website still never grants administrator access.
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The copy has to be the *same* MSYS2 installation, not a second one: `C:\msys2\opt\kik` is two levels below
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`C:\msys2`, so its runtime resolves the same root and the login shell and the user's own tools share one cygwin
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runtime. A separate installation does not work - two runtimes in one process tree corrupt each other's file
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descriptors and pacman cannot even lock its database.
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The runtime itself is not among the protected copies. A program and the shell that starts it have to use the same
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runtime DLL, or the program does not run at all - a second copy of it in one process is refused. The login path
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therefore uses the one in `/usr/bin`, next to the user's own programs, and the user may replace and upgrade it.
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What is protected instead is that it is the one the package says it is: pacman checks the files of `msys2-runtime`,
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and the cleanup job puts the package back if it was changed - while the server is stopped, before it starts it
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again, because the server runs as SYSTEM and would otherwise run code the user of the website wrote.
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`/etc/passwd` (the login shell and home directory) and `/etc/ssh` (the host keys and config) are read-only to the
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user as well; everything else in the installation is theirs.
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## The cleanup tasks
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Three scheduled tasks run as SYSTEM, all at startup, and the user cannot stop any of them:
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- `KiK sshd` runs the sshd from `C:\msys2\opt\kik`. It replaces the installation's own `msys2-sshd` service, which
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is disabled.
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- `KiK restore shell files` runs every 30 minutes and rebuilds the protected copy from its trusted originals,
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re-applies the permissions, and puts the home directory, the shell configuration, the website's key and the four
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texts back. Its second job is to best-effort revert other mockery and bring a fresh server to the expected state.
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- `KiK watch the server` runs every two minutes and starts the server when nothing listens on port 22. The task of
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the server only starts it again when it failed, never when it was stopped, so a cleanup job that is killed while
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it has the server stopped would otherwise take the site down with it.
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The cleanup job only restarts the server when it has to: when it does not listen at all, when it is older than its
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configuration, or when the protected copy or the runtime has to be repaired - those cannot be replaced while the
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server holds them open. A working server is left alone, because every start can fail, and failed starts have kept
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the site down for hours. The job also never restarts the server while somebody is connected: a session that is open
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keeps the port, so a server that is started then could not bind it.
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The server is only down for the short part of the job that needs it down: the protected copy cannot be replaced
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while the server holds it open. Everything else - the home directory, the four texts, the permissions of the
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installation - is put back while the server runs, because on a slow disk that takes minutes and the site should
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have a shell the whole time.
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The task that runs the server ignores a start while it thinks it is still running, so a start that is dropped
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leaves the site without a shell. The cleanup job therefore tries three times, killing the processes of the task
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in between, before it gives up.
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The deployment does not restart the server. It runs in a session, and that session keeps the port, so a server that
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starts during a deployment cannot bind it. The new configuration and the restored programs therefore take effect
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at the next run of the cleanup job, which happens within half an hour.
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The server logs to `C:\ProgramData\kik-restore\sshd.log`. The log is appended to, not overwritten, because it is
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the only trace of what the server did: a server that is restarted would otherwise take the evidence of an outage
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with it. It is rotated when it grows past a hundred megabytes, which takes years. The event logs of windows are
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made bigger as well, the security log to a gigabyte: the scanners of the remote desktop alone fill a hundred
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megabytes of it in a week.
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A start of the server that fails leaves nothing in that log. So windows records the history of the scheduled tasks
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and the start and end of every process, and when the server does not come up, the cleanup job writes the processes
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and the state of the server's task to `C:\ProgramData\kik-restore\failed-starts.log`.
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Only keys may log in over SSH. The box stands in the open internet and both of its accounts have a password.
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The files that have to stay belong to the administrators, so the user cannot delete them, and they cannot change the
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shell configuration or the key the website logs in with either. The shell reads its configuration from `~/.zsh`, so
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the startup files the user could write in the home directory are ignored.
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## Installing the cleanup on the server
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To deploy the cleanup task an the server, run:
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server/scripts/deploy.sh
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The script logs in as `Administrator` - whose shell is a Windows one - copies the scripts and the resources to the
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Administrator's home directory and runs the installer there.
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