kik-website/server/scripts/install_restore_task.ps1

# Installs the scheduled task that puts the shell server's home directory back into shape every 30 minutes: the
# prompt, the home directory and the four text files. The task runs as SYSTEM, so the shell server's root user can
# neither stop it nor change the copies it restores from.
#
# Run this in an elevated PowerShell on the server, with this script and the ../resources directory next to it:
#   powershell -ExecutionPolicy Bypass -File .\install_restore_task.ps1
# Remove the task and the copies again with:
#   powershell -ExecutionPolicy Bypass -File .\install_restore_task.ps1 -Uninstall
[CmdletBinding()]
param(
    [string]$HomeDir = 'C:\msys2\home\root',
    [switch]$Uninstall
)

$ErrorActionPreference = 'Stop'
$installDir = 'C:\ProgramData\kik-restore'
$copiesDir = Join-Path $installDir 'files'
$taskName = 'KiK restore shell files'

# The copies the task restores from and the script it runs both live in $installDir, which only SYSTEM and the
# administrators may read or write.
$restoreScript = @'
# Puts the shell server's home directory back into shape: the directory itself, the shell configuration that gives
# the prompt, the keys that let the website in, and the four text files. Runs as SYSTEM every 30 minutes; the shell
# server's root user can neither stop it nor change the copies it restores from.
param([string]$HomeDir = 'C:\msys2\home\root')
$log = Join-Path $PSScriptRoot 'restore.log'
$copies = Join-Path $PSScriptRoot 'files'

# The account of the shell server's root user, whose id the files in its home directory belong to.
$passwd = Get-Content 'C:\msys2\etc\passwd' -ErrorAction SilentlyContinue | Where-Object { $_ -like 'root:*' }
$sid = if ($passwd) { ($passwd -split ':')[4].Split(',')[-1] }

function Write-Log($message) {
    "$(Get-Date -Format s) $message" | Out-File -FilePath $log -Append
}

# Removes a file, a directory or a link, but never what a link points to.
function Remove-Anything($path) {
    $item = Get-Item -LiteralPath $path -Force
    if ($item.Attributes -band [IO.FileAttributes]::ReparsePoint) {
        if ($item.PSIsContainer) { [IO.Directory]::Delete($path) } else { [IO.File]::Delete($path) }
    } elseif ($item.PSIsContainer) {
        Remove-Item -LiteralPath $path -Recurse -Force
    } else {
        Remove-Item -LiteralPath $path -Force
    }
}

# The website logs in with a key in the authorized_keys of the user. The user may keep keys of their own, so only
# the key of the website is put back.
function Restore-Keys($target, $source) {
    $required = @(Get-Content -LiteralPath $source | Where-Object { $_.Trim() })
    $blob = ($required[0] -split '\s+')[1]
    $keys = @()
    if (Test-Path -LiteralPath $target) {
        $keys = @(Get-Content -LiteralPath $target | Where-Object { $_.Trim() -and ($_ -split '\s+')[1] -ne $blob })
    }
    [IO.File]::WriteAllText($target, (($keys + $required) -join "`n") + "`n")
}

# The home directory is the one from /etc/passwd, which the user cannot change, but which they can delete. The
# directories in it belong to the user, so that they can put their own keys in .ssh.
function New-Dir($path) {
    New-Item -ItemType Directory -Path $path | Out-Null
    # The parent directory gives the users read access only, so the owner gets write access on their own home.
    if ($sid) {
        icacls $path /grant "*${sid}:(OI)(CI)F" | Out-Null
        icacls $path /setowner "*$sid" | Out-Null
    }
}

if (-not (Test-Path -LiteralPath $HomeDir)) {
    New-Dir $HomeDir
    Write-Log "created $HomeDir"
} elseif ((Get-Item -LiteralPath $HomeDir -Force).Attributes -band [IO.FileAttributes]::ReparsePoint) {
    Remove-Anything $HomeDir
    New-Dir $HomeDir
    Write-Log "replaced the link $HomeDir"
}

foreach ($copy in Get-ChildItem -LiteralPath $copies -Recurse -File -Force) {
    $target = Join-Path $HomeDir $copy.FullName.Substring($copies.Length).TrimStart('\')
    try {
        $dir = Split-Path -Parent $target
        if ((Test-Path -LiteralPath $dir) -and -not (Get-Item -LiteralPath $dir -Force).PSIsContainer) {
            Remove-Anything $dir
        }
        if (-not (Test-Path -LiteralPath $dir)) { New-Item -ItemType Directory -Path $dir | Out-Null }
        if ($sid) {
            icacls $dir /grant "*${sid}:(OI)(CI)F" | Out-Null
            icacls $dir /setowner "*$sid" | Out-Null
        }
        if (Test-Path -LiteralPath $target) {
            $item = Get-Item -LiteralPath $target -Force
            if ($item.PSIsContainer -or $item.Attributes -band [IO.FileAttributes]::ReparsePoint) {
                # anything but a file is replaced, so that the files are always the text files they should be
                Remove-Anything $target
            } else {
                # the user may have locked the file; taking it back lets us write it
                takeown /f $target /a | Out-Null
                icacls $target /reset | Out-Null
            }
        }
        if ($copy.Name -eq 'authorized_keys') {
            Restore-Keys $target $copy.FullName
        } else {
            Copy-Item -LiteralPath $copy.FullName -Destination $target -Force
        }
        if ($sid) { icacls $target /setowner "*$sid" | Out-Null }
        Write-Log "restored $($copy.Name)"
    } catch {
        Write-Log "failed $($copy.Name) : $_"
    }
}
'@

if (-not ([Security.Principal.WindowsPrincipal] [Security.Principal.WindowsIdentity]::GetCurrent()).IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)) {
    throw 'Run this in an elevated PowerShell.'
}

if ($Uninstall) {
    Unregister-ScheduledTask -TaskName $taskName -Confirm:$false -ErrorAction SilentlyContinue
    if (Test-Path -LiteralPath $installDir) { Remove-Item -LiteralPath $installDir -Recurse -Force }
    Write-Host "Removed the task and $installDir."
    return
}

$resources = Join-Path $PSScriptRoot '..\resources'
if (-not (Get-ChildItem -LiteralPath $resources -File -Force)) { throw "No files in $resources." }

# The root user may have prepared the directory or the file names, so nothing that looks like a link is used.
$prepared = Get-Item -LiteralPath $installDir -Force -ErrorAction SilentlyContinue
if ($prepared -and $prepared.Attributes -band [IO.FileAttributes]::ReparsePoint) { throw "$installDir is a link, refusing to install." }

New-Item -ItemType Directory -Force -Path $copiesDir | Out-Null

# Only SYSTEM and the administrators may read or write the copies and the script.
$acl = New-Object System.Security.AccessControl.DirectorySecurity
$acl.SetAccessRuleProtection($true, $false)
foreach ($sid in 'S-1-5-18', 'S-1-5-32-544') {
    $identity = New-Object System.Security.Principal.SecurityIdentifier $sid
    $acl.AddAccessRule((New-Object System.Security.AccessControl.FileSystemAccessRule -ArgumentList $identity, 'FullControl', 'ContainerInherit,ObjectInherit', 'None', 'Allow'))
}
$acl.SetOwner((New-Object System.Security.Principal.SecurityIdentifier 'S-1-5-32-544'))
Set-Acl -LiteralPath $installDir -AclObject $acl

# Files the root user prepared are removed, the copies are written fresh and inherit the restricted permissions.
foreach ($item in Get-ChildItem -LiteralPath $copiesDir -Recurse -Force) {
    if ($item.Attributes -band [IO.FileAttributes]::ReparsePoint) { throw "$($item.FullName) is a link, refusing to install." }
}
Remove-Item -LiteralPath $copiesDir -Recurse -Force
New-Item -ItemType Directory -Force -Path $copiesDir | Out-Null
Copy-Item -Path (Join-Path $resources '*') -Destination $copiesDir -Recurse -Force

# The Administrator logs in with the same key, so that the deployment keeps working. The keys that are already
# there stay.
$adminKeys = 'C:\Users\Administrator\.ssh\authorized_keys'
$required = @(Get-Content -LiteralPath (Join-Path $resources '.ssh\authorized_keys') | Where-Object { $_.Trim() })
$keys = @()
if (Test-Path -LiteralPath $adminKeys) { $keys = @(Get-Content -LiteralPath $adminKeys | Where-Object { $_.Trim() }) }
if (-not ($keys | Where-Object { ($_ -split '\s+')[1] -eq ($required[0] -split '\s+')[1] })) {
    New-Item -ItemType Directory -Force -Path (Split-Path -Parent $adminKeys) | Out-Null
    [IO.File]::WriteAllText($adminKeys, (($keys + $required) -join "`n") + "`n")
}
[IO.File]::WriteAllText((Join-Path $installDir 'restore.ps1'), $restoreScript)

$action = New-ScheduledTaskAction -Execute 'powershell.exe' -Argument "-NoProfile -NonInteractive -ExecutionPolicy Bypass -WindowStyle Hidden -File `"$installDir\restore.ps1`" -HomeDir `"$HomeDir`""
$triggers = @(
    New-ScheduledTaskTrigger -Once -At (Get-Date) -RepetitionInterval (New-TimeSpan -Minutes 30) -RepetitionDuration (New-TimeSpan -Days 3650)
    New-ScheduledTaskTrigger -AtStartup
)
$settings = New-ScheduledTaskSettingsSet -StartWhenAvailable -MultipleInstances IgnoreNew -ExecutionTimeLimit (New-TimeSpan -Minutes 10) -RestartCount 3 -RestartInterval (New-TimeSpan -Minutes 5) -AllowStartIfOnBatteries -DontStopIfGoingOnBatteries
$principal = New-ScheduledTaskPrincipal -UserId 'SYSTEM' -LogonType ServiceAccount -RunLevel Highest
Register-ScheduledTask -TaskName $taskName -Action $action -Trigger $triggers -Principal $principal -Settings $settings -Force | Out-Null

Start-ScheduledTask -TaskName $taskName
Start-Sleep -Seconds 5
Write-Host "Task '$taskName' runs as SYSTEM every 30 minutes and at startup."
Get-ScheduledTaskInfo -TaskName $taskName | Format-List LastRunTime, LastTaskResult, NextRunTime
Get-Content (Join-Path $installDir 'restore.log') -Tail 8 -ErrorAction SilentlyContinue
icacls $installDir