Backport SSH host key detection code to CE

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

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

app/models/ssh_host_key.rb

+# 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

config/routes/project.rb

@@ -178,6 +178,7 @@ constraints(::Constraints::ProjectUrlConstrainer.new) do
 
       resource :mirror, only: [:show, :update] do
         member do
+          get :ssh_host_keys, constraints: { format: :json }
           post :update_now
         end
       end

spec/controllers/projects/mirrors_controller_spec.rb

@@ -63,6 +63,69 @@ describe Projects::MirrorsController do
     end
   end
 
+  describe '#ssh_host_keys', :use_clean_rails_memory_store_caching do
+    let(:project) { create(:project) }
+    let(:cache) { SshHostKey.new(project: project, url: "ssh://example.com:22") }
+
+    before do
+      sign_in(project.owner)
+    end
+
+    context 'invalid URLs' do
+      %w[
+        INVALID
+        git@example.com:foo/bar.git
+        ssh://git@example.com:foo/bar.git
+      ].each do |url|
+        it "returns an error with a 400 response for URL #{url.inspect}" do
+          do_get(project, url)
+
+          expect(response).to have_gitlab_http_status(400)
+          expect(json_response).to eq('message' => 'Invalid URL')
+        end
+      end
+    end
+
+    context 'no data in cache' do
+      it 'requests the cache to be filled and returns a 204 response' do
+        expect(ReactiveCachingWorker).to receive(:perform_async).with(cache.class, cache.id).at_least(:once)
+
+        do_get(project)
+
+        expect(response).to have_gitlab_http_status(204)
+      end
+    end
+
+    context 'error in the cache' do
+      it 'returns the error with a 400 response' do
+        stub_reactive_cache(cache, error: 'An error')
+
+        do_get(project)
+
+        expect(response).to have_gitlab_http_status(400)
+        expect(json_response).to eq('message' => 'An error')
+      end
+    end
+
+    context 'data in the cache' do
+      let(:ssh_key) { 'ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAfuCHKVTjquxvt6CM6tdG4SLp1Btn/nOeHHE5UOzRdf' }
+      let(:ssh_fp) { { type: 'ed25519', bits: 256, fingerprint: '2e:65:6a:c8:cf:bf:b2:8b:9a:bd:6d:9f:11:5c:12:16', index: 0 } }
+
+      it 'returns the data with a 200 response' do
+        stub_reactive_cache(cache, known_hosts: ssh_key)
+
+        do_get(project)
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response).to eq('known_hosts' => ssh_key, 'fingerprints' => [ssh_fp.stringify_keys], 'host_keys_changed' => true)
+      end
+    end
+
+    def do_get(project, url = 'ssh://example.com')
+      get :ssh_host_keys, namespace_id: project.namespace, project_id: project, ssh_url: url
+    end
+  end
+
   def do_put(project, options, extra_attrs = {})
     attrs = extra_attrs.merge(namespace_id: project.namespace.to_param, project_id: project.to_param)
     attrs[:project] = options

spec/models/ssh_host_key_spec.rb

+require 'spec_helper'
+
+describe SshHostKey do
+  using RSpec::Parameterized::TableSyntax
+  include ReactiveCachingHelpers
+
+  let(:key1) do
+    'ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC3UpyF2iLqy1d63M6k3jH1vuEnq/NWtE+o' \
+    'rJe1Xn7JoRbduKd6zpsJ0JhBGWgcQK0ph0aGW5PcudzzBSc+SlYfCc4GTaxDtmj41hW0o72m' \
+    'NiuDW3oKXXShOiVRde2ZOquH8Z865jGiZIC8BI/bXZD29IGUih0hPu7Rjp70VYiE+35QRf/p' \
+    'sD0Ddrz8QUIG3A/2dMzLI5F5ZORk3BIX2F3mJwJOvZxRhR/SqyphDMZ5eZ0EzqbFBCDE6HAB' \
+    'Woz9ck8RBGLvCIggmDHj3FmMLcQGMDiy6wKp7QdnBtxjCP6vtE6YPUM223AqsWt+9NTtCfB8' \
+    'YdNAH7YcHHOR1FgtSk1x'
+  end
+
+  let(:key2) do
+    'ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDLIp+4ciR2YO9f9rpldc7InNQw/TBUtcNb' \
+    'J2XR0rr15/5ytz7YM16xXG0Qjx576PNSmqs4gbTrvTuFZak+v1Jx/9deHRq/yqp9f+tv33+i' \
+    'aJGCQCX/+OVY7aWgV2R9YsS7XQ4mnv4XlOTEssib/rGAIT+ATd/GcdYSEOO+dh4O09/6O/jI' \
+    'MGSeP+NNetgn1nPCnLOjrXFZUnUtNDi6EEKeIlrliJjSb7Jr4f7gjvZnv4RskWHHFo8FgAAq' \
+    't0gOMT6EmKrnypBe2vLGSAXbtkXr01q6/DNPH+n9VA1LTV6v1KN/W5CN5tQV11wRSKiM8g5O' \
+    'Ebi86VjJRi2sOuYoXQU1'
+  end
+
+  # Purposefully ordered so that `sort` will make changes
+  let(:known_hosts) do
+    <<~EOF
+      example.com #{key1} git@localhost
+      @revoked other.example.com #{key2} git@localhost
+    EOF
+  end
+
+  let(:extra) { known_hosts + "foo\nbar\n" }
+  let(:reversed) { known_hosts.lines.reverse.join }
+
+  let(:compare_host_keys) { nil }
+
+  def stub_ssh_keyscan(args, status: true, stdout: "", stderr: "")
+    stdin = StringIO.new
+    stdout = double(:stdout, read: stdout)
+    stderr = double(:stderr, read: stderr)
+    wait_thr = double(:wait_thr, value: double(success?: status))
+
+    expect(Open3).to receive(:popen3).with({}, 'ssh-keyscan', *args).and_yield(stdin, stdout, stderr, wait_thr)
+
+    stdin
+  end
+
+  let(:project) { build(:project) }
+
+  subject(:ssh_host_key) { described_class.new(project: project, url: 'ssh://example.com:2222', compare_host_keys: compare_host_keys) }
+
+  describe '#fingerprints', :use_clean_rails_memory_store_caching do
+    it 'returns an array of indexed fingerprints when the cache is filled' do
+      stub_reactive_cache(ssh_host_key, known_hosts: known_hosts)
+
+      expected = [key1, key2]
+        .map { |data| Gitlab::SSHPublicKey.new(data) }
+        .each_with_index
+        .map { |key, i| { bits: key.bits, fingerprint: key.fingerprint, type: key.type, index: i } }
+
+      expect(ssh_host_key.fingerprints.as_json).to eq(expected)
+    end
+
+    it 'returns an empty array when the cache is empty' do
+      expect(ssh_host_key.fingerprints).to eq([])
+    end
+  end
+
+  describe '#fingerprints', :use_clean_rails_memory_store_caching do
+    it 'returns an array of indexed fingerprints when the cache is filled' do
+      stub_reactive_cache(ssh_host_key, known_hosts: known_hosts)
+
+      expect(ssh_host_key.fingerprints.as_json).to eq(
+        [
+          { bits: 2048, fingerprint: Gitlab::SSHPublicKey.new(key1).fingerprint, type: :rsa, index: 0 },
+          { bits: 2048, fingerprint: Gitlab::SSHPublicKey.new(key2).fingerprint, type: :rsa, index: 1 }
+        ]
+      )
+    end
+
+    it 'returns an empty array when the cache is empty' do
+      expect(ssh_host_key.fingerprints).to eq([])
+    end
+  end
+
+  describe '#host_keys_changed?' do
+    where(:known_hosts_a, :known_hosts_b, :result) do
+      known_hosts | extra       | true
+      known_hosts | "foo\n"     | true
+      known_hosts | ''          | true
+      known_hosts | nil         | true
+      known_hosts | known_hosts | false
+      reversed    | known_hosts | false
+      extra       | "foo\n"     | true
+      ''          | ''          | false
+      nil         | nil         | false
+      ''          | nil         | false
+    end
+
+    with_them do
+      let(:compare_host_keys) { known_hosts_b }
+
+      subject { ssh_host_key.host_keys_changed? }
+
+      context '(normal)' do
+        let(:compare_host_keys) { known_hosts_b }
+
+        before do
+          expect(ssh_host_key).to receive(:known_hosts).and_return(known_hosts_a)
+        end
+
+        it { is_expected.to eq(result) }
+      end
+
+      # Comparisons should be symmetrical, so test the reverse too
+      context '(reversed)' do
+        let(:compare_host_keys) { known_hosts_a }
+
+        before do
+          expect(ssh_host_key).to receive(:known_hosts).and_return(known_hosts_b)
+        end
+
+        it { is_expected.to eq(result) }
+      end
+    end
+  end
+
+  describe '#calculate_reactive_cache' do
+    subject(:cache) { ssh_host_key.calculate_reactive_cache }
+
+    it 'writes the hostname to STDIN' do
+      stdin = stub_ssh_keyscan(%w[-T 5 -p 2222 -f-])
+
+      cache
+
+      expect(stdin.string).to eq("example.com\n")
+    end
+
+    context 'successful key scan' do
+      it 'stores the cleaned known_hosts data' do
+        stub_ssh_keyscan(%w[-T 5 -p 2222 -f-], stdout: "KEY 1\nKEY 1\n\n# comment\nKEY 2\n")
+
+        is_expected.to eq(known_hosts: "KEY 1\nKEY 2\n")
+      end
+    end
+
+    context 'failed key scan (exit code 1)' do
+      it 'returns a generic error' do
+        stub_ssh_keyscan(%w[-T 5 -p 2222 -f-], stdout: 'blarg', status: false)
+
+        is_expected.to eq(error: 'Failed to detect SSH host keys')
+      end
+    end
+
+    context 'failed key scan (exit code 0)' do
+      it 'returns a generic error' do
+        stub_ssh_keyscan(%w[-T 5 -p 2222 -f-], stderr: 'Unknown host')
+
+        is_expected.to eq(error: 'Failed to detect SSH host keys')
+      end
+    end
+  end
+end