Backport SSH host key detection code to CE

This functionality is needed for SSH push mirroring support, which is a
CE feature.

2 files changed, 146 insertions, 0 deletions

diff --git a/app/controllers/projects/mirrors_controller.rb b/app/controllers/projects/mirrors_controller.rb
index 78d5faf2326..53176978416 100644
--- a/app/controllers/projects/mirrors_controller.rb
+++ b/app/controllers/projects/mirrors_controller.rb
@@ -44,6 +44,22 @@ class Projects::MirrorsController < Projects::ApplicationController
     redirect_to_repository_settings(project, anchor: 'js-push-remote-settings')
   end
 
+  def ssh_host_keys
+    lookup = SshHostKey.new(project: project, url: params[:ssh_url], compare_host_keys: params[:compare_host_keys])
+
+    if lookup.error.present?
+      # Failed to read keys
+      render json: { message: lookup.error }, status: :bad_request
+    elsif lookup.known_hosts.nil?
+      # Still working, come back later
+      render body: nil, status: :no_content
+    else
+      render json: lookup
+    end
+  rescue ArgumentError => err
+    render json: { message: err.message }, status: :bad_request
+  end
+
   private
 
   def remote_mirror
diff --git a/app/models/ssh_host_key.rb b/app/models/ssh_host_key.rb
new file mode 100644
index 00000000000..b6844dbe870
--- /dev/null
+++ b/app/models/ssh_host_key.rb
@@ -0,0 +1,130 @@
+# frozen_string_literal: true
+
+# Detected SSH host keys are transiently stored in Redis
+class SshHostKey
+  class Fingerprint < Gitlab::SSHPublicKey
+    attr_reader :index
+
+    def initialize(key, index: nil)
+      super(key)
+
+      @index = index
+    end
+
+    def as_json(*)
+      { bits: bits, fingerprint: fingerprint, type: type, index: index }
+    end
+  end
+
+  include ReactiveCaching
+
+  self.reactive_cache_key = ->(key) { [key.class.to_s, key.id] }
+
+  # Do not refresh the data in the background - it is not expected to change.
+  # This is achieved by making the lifetime shorter than the refresh interval.
+  self.reactive_cache_refresh_interval = 15.minutes
+  self.reactive_cache_lifetime = 10.minutes
+
+  def self.find_by(opts = {})
+    return nil unless opts.key?(:id)
+
+    project_id, url = opts[:id].split(':', 2)
+    project = Project.find_by(id: project_id)
+
+    project.presence && new(project: project, url: url)
+  end
+
+  def self.fingerprint_host_keys(data)
+    return [] unless data.is_a?(String)
+
+    data
+      .each_line
+      .each_with_index
+      .map { |line, index| Fingerprint.new(line, index: index) }
+      .select(&:valid?)
+  end
+
+  attr_reader :project, :url, :compare_host_keys
+
+  def initialize(project:, url:, compare_host_keys: nil)
+    @project = project
+    @url = normalize_url(url)
+    @compare_host_keys = compare_host_keys
+  end
+
+  def id
+    [project.id, url].join(':')
+  end
+
+  def as_json(*)
+    {
+      host_keys_changed: host_keys_changed?,
+      fingerprints: fingerprints,
+      known_hosts: known_hosts
+    }
+  end
+
+  def known_hosts
+    with_reactive_cache { |data| data[:known_hosts] }
+  end
+
+  def fingerprints
+    @fingerprints ||= self.class.fingerprint_host_keys(known_hosts)
+  end
+
+  # Returns true if the known_hosts data differs from the version passed in at
+  # initialization as `compare_host_keys`. Comments, ordering, etc, is ignored
+  def host_keys_changed?
+    cleanup(known_hosts) != cleanup(compare_host_keys)
+  end
+
+  def error
+    with_reactive_cache { |data| data[:error] }
+  end
+
+  def calculate_reactive_cache
+    known_hosts, errors, status =
+      Open3.popen3({}, *%W[ssh-keyscan -T 5 -p #{url.port} -f-]) do |stdin, stdout, stderr, wait_thr|
+        stdin.puts(url.host)
+        stdin.close
+
+        [
+          cleanup(stdout.read),
+          cleanup(stderr.read),
+          wait_thr.value
+        ]
+      end
+
+    # ssh-keyscan returns an exit code 0 in several error conditions, such as an
+    # unknown hostname, so check both STDERR and the exit code
+    if status.success? && !errors.present?
+      { known_hosts: known_hosts }
+    else
+      Rails.logger.debug("Failed to detect SSH host keys for #{id}: #{errors}")
+
+      { error: 'Failed to detect SSH host keys' }
+    end
+  end
+
+  private
+
+  # Remove comments and duplicate entries
+  def cleanup(data)
+    data
+      .to_s
+      .each_line
+      .reject { |line| line.start_with?('#') || line.chomp.empty? }
+      .uniq
+      .sort
+      .join
+  end
+
+  def normalize_url(url)
+    full_url = ::Addressable::URI.parse(url)
+    raise ArgumentError.new("Invalid URL") unless full_url&.scheme == 'ssh'
+
+    Addressable::URI.parse("ssh://#{full_url.host}:#{full_url.inferred_port}")
+  rescue Addressable::URI::InvalidURIError
+    raise ArgumentError.new("Invalid URL")
+  end
+end