Don't send SameSite=None to incompatible browsers

We set `SameSite=None` in GitLab 12.10 via
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/28205 because
Chrome v80, rolled out in March 2020, treats any cookies without the
`SameSite` directive set as though they are `SameSite=Lax`
(https://www.chromestatus.com/feature/5088147346030592). This is a
breaking change from the previous default behavior, which was to treat
those cookies as `SameSite=None`.

However, older browsers (e.g. MacOS 10.14 on Safari 13.0.3) may
interpret the `None` as `Strict`, which causes users that click on
gitlab.com links from third-party sites (e.g. Gmail, Slack) to log in
again.

https://www.chromium.org/updates/same-site/incompatible-clients
recommends a set of regular expressions to determine whether to send
this. This commit implements most of the logic but skips one case since
this doesn't seem common: macOS 10.14 with an embedded WebKit
browser. This is also what
https://rubygems.org/gems/rails_same_site_cookie does.

I considered using that gem
(https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40663), but I
didn't like how it added another dependency (`user_agent_parser`) that
loads a large YAML database
(https://github.com/ua-parser/uap-ruby#the-pattern-database).

changelogs/unreleased/sh-same-site-none-fix-try2.yml

+---
+title: Don't send SameSite=None to incompatible browsers
+merge_request: 40667
+author:
+type: fixed

lib/gitlab/middleware/same_site_cookies.rb

@@ -33,6 +33,8 @@ module Gitlab
 
         cookies = set_cookie.split(COOKIE_SEPARATOR)
 
+        return result if same_site_none_incompatible?(headers['User-Agent'])
+
         cookies.each do |cookie|
           next if cookie.blank?
 
@@ -58,6 +60,23 @@ module Gitlab
       def ssl?
         Gitlab.config.gitlab.https
       end
+
+      # Taken from https://www.chromium.org/updates/same-site/incompatible-clients
+      def same_site_none_incompatible?(user_agent)
+        return false unless user_agent.present?
+
+        browser = Browser.new(user_agent)
+        has_webkit_same_site_bug?(browser) || drops_unrecognized_same_site_cookies?(browser)
+      end
+
+      def has_webkit_same_site_bug?(browser)
+        browser.platform.ios?(12) ||
+          (browser.platform.mac?("~> 10.14") && browser.safari?)
+      end
+
+      def drops_unrecognized_same_site_cookies?(browser)
+        browser.uc_browser?("<12.13.2") || browser.chrome?([">= 51", "< 67"])
+      end
     end
   end
 end

spec/lib/gitlab/middleware/same_site_cookies_spec.rb

@@ -3,23 +3,30 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::Middleware::SameSiteCookies do
+  using RSpec::Parameterized::TableSyntax
   include Rack::Test::Methods
 
+  let(:user_agent) { nil }
   let(:mock_app) do
     Class.new do
-      attr_reader :cookies
+      attr_reader :cookies, :user_agent
 
-      def initialize(cookies)
+      def initialize(cookies, user_agent)
         @cookies = cookies
+        @user_agent = user_agent
       end
 
       def call(env)
-        [200, { 'Set-Cookie' => cookies }, ['OK']]
+        [
+          200,
+          { 'Set-Cookie' => cookies, 'User-Agent' => user_agent }.compact,
+          ['OK']
+        ]
       end
     end
   end
 
-  let(:app) { mock_app.new(cookies) }
+  let(:app) { mock_app.new(cookies, user_agent) }
 
   subject do
     described_class.new(app)
@@ -63,6 +70,42 @@ RSpec.describe Gitlab::Middleware::SameSiteCookies do
         end
       end
 
+      context 'with different browsers' do
+        where(:description, :user_agent, :expected) do
+          "iOS 12" | "Mozilla/5.0 (iPhone; CPU iPhone OS 12_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0 Mobile/15E148 Safari/604.1" | false
+          "macOS 10.14 + Safari" | "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0 Safari/605.1.15" | false
+          "macOS 10.14 + Opera" | "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36 OPR/47.0.2631.55" | false
+          "macOS 10.14 + Chrome v80" | "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.87 Safari/537.36" | true
+          "Chrome v41" | "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.1 Safari/537.36" | true
+          "Chrome v50" | "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2348.1 Safari/537.36" | true
+          "Chrome v51" | "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2718.15 Safari/537.36" | false
+          "Chrome v66" | "Mozilla/5.0 (Linux; Android 4.4.2; Avvio_793 Build/KOT49H) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.126 Mobile Safari/537.36" | false
+          "Chrome v67" | "Mozilla/5.0 (Linux; Android 7.1.1; SM-J510F Build/NMF26X) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3371.0 Mobile Safari/537.36" | true
+          "Chrome v85" | "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.83 Safari/537.36" | true
+          "Chromium v66" | "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/66.0.3359.181 HeadlessChrome/66.0.3359.181 Safari/537.36" | false
+          "Chromium v85" | "Mozilla/5.0 (X11; Linux aarch64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/85.0.4183.59 Chrome/85.0.4183.59 Safari/537.36" | true
+          "UC Browser 12.0.4" | "Mozilla/5.0 (Linux; U; Android 4.4.4; zh-CN; A31 Build/KTU84P) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/57.0.2987.108 UCBrowser/12.0.4.986 Mobile Safari/537.36" | false
+          "UC Browser 12.13.0" | "Mozilla/5.0 (Linux; U; Android 7.1.1; en-US; SM-C9000 Build/NMF26X) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/57.0.2987.108 UCBrowser/12.13.0.1207 Mobile Safari/537.36" | false
+          "UC Browser 12.13.2" | "Mozilla/5.0 (Linux; U; Android 9; en-US; Redmi Note 7 Build/PQ3B.190801.002) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/57.0.2987.108 UCBrowser/12.13.2.1208 Mobile Safari/537.36" | true
+          "UC Browser 12.13.5" | "Mozilla/5.0 (Linux; U; Android 5.1.1; en-US; PHICOMM C630 (CLUE L) Build/LMY47V) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/57.0.2987.108 UCBrowser/12.13.5.1209 Mobile Safari/537.36" | true
+          "Playstation" | "Mozilla/5.0 (PlayStation 4 2.51) AppleWebKit/537.73 (KHTML, like Gecko)" | true
+        end
+
+        with_them do
+          let(:cookies) { "thiscookie=12345" }
+
+          it 'returns expected SameSite status' do
+            response = do_request
+
+            if expected
+              expect(response['Set-Cookie']).to include('SameSite=None')
+            else
+              expect(response['Set-Cookie']).not_to include('SameSite=None')
+            end
+          end
+        end
+      end
+
       context 'with single cookie' do
         let(:cookies) { "thiscookie=12345" }