Fix BadCSRFToken error

Fix a `BadCSRFToken` error that occurs when users leave a page open for
more than an hour and then try to continue using it.

Fixes #8940 (see the issue for
details of the bug and steps to reproduce).

h/security/policy/top_level.py

@@ -11,6 +11,9 @@
 from h.security.policy.helpers import AuthTicketCookieHelper, is_api_request
 
 
+HTML_AUTHCOOKIE_MAX_AGE = 30 * 24 * 3600  # 30 days.
+
+
 class TopLevelPolicy:
     """The top-level security policy. Delegates to subpolicies."""
 
@@ -42,8 +45,6 @@ def _load_identity(self, request):
 def get_subpolicy(request):
     """Return the subpolicy for TopLevelSecurityPolicy to delegate to for `request`."""
 
-    html_authcookie_max_age = 30 * 24 * 3600  # 30 days.
-
     # The cookie that's used to authenticate API requests.
     api_authcookie = webob.cookies.SignedCookieProfile(
         secret=request.registry.settings["h_api_auth_cookie_secret"],
@@ -55,7 +56,7 @@ def get_subpolicy(request):
         # Make the API authcookie stay fresh for longer than the HTML one.
         # This is to make it less likely that a browser will have an unexpired HTML
         # authcookie but an expired API one, which can lead to confusing results.
-        max_age=2 * html_authcookie_max_age,
+        max_age=2 * HTML_AUTHCOOKIE_MAX_AGE,
     )
     api_authcookie = api_authcookie.bind(request)
 
@@ -64,7 +65,7 @@ def get_subpolicy(request):
         secret=request.registry.settings["h_auth_cookie_secret"],
         salt="authsanity",
         cookie_name="auth",
-        max_age=html_authcookie_max_age,
+        max_age=HTML_AUTHCOOKIE_MAX_AGE,
         httponly=True,
         secure=request.scheme == "https",
     )

h/session.py

@@ -4,6 +4,7 @@
 from pyramid.session import JSONSerializer, SignedCookieSessionFactory
 
 from h.security import derive_key
+from h.security.policy.top_level import HTML_AUTHCOOKIE_MAX_AGE
 
 
 def model(request):
@@ -110,7 +111,22 @@ def includeme(config):  # pragma: no cover
         httponly=True,
         secure=secure,
         serializer=JSONSerializer(),
-        timeout=3600,
+        # One thing h uses the session for is to store CSRF tokens (see
+        # SessionCSRFStoragePolicy() below).
+        #
+        # The auth cookies that keep users logged in to h web pages have a
+        # lifetime of HTML_AUTHCOOKIE_MAX_AGE so in theory a user can leave a
+        # tab open (say a page containing a form) for up to
+        # HTML_AUTHCOOKIE_MAX_AGE and then return to the tab and expect to be
+        # able to submit the form.
+        #
+        # However, even if the user's auth cookie is still valid their form
+        # submission will still fail if the form's CSRF token has expired and
+        # the user will see a BadCSRFToken error.
+        #
+        # To avoid this we make sure that the lifetime of CSRF tokens is always
+        # longer than the lifetimes of auth cookies.
+        timeout=2*HTML_AUTHCOOKIE_MAX_AGE,
     )
     config.set_session_factory(factory)
     config.set_csrf_storage_policy(SessionCSRFStoragePolicy())