Keep subdomains of an isolated origin in the isolated origin's SiteInstance.

content/browser/site_instance_impl.cc

Index: content/browser/site_instance_impl.cc
diff --git a/content/browser/site_instance_impl.cc b/content/browser/site_instance_impl.cc
index 0e47c20885ad4e5e8b7396062cf192290c32b3fb..4a6424a9ce96b9d347b7f170d150fb72f97e68d0 100644
--- a/content/browser/site_instance_impl.cc
+++ b/content/browser/site_instance_impl.cc
@@ -309,13 +309,25 @@ bool SiteInstance::IsSameWebSite(BrowserContext* browser_context,
   if (dest_url == blank_page)
     return true;
 
-  // If either URL has an isolated origin, compare origins rather than sites.
+  // If either URL matches an isolated origin, compare origins rather than
+  // sites.
   url::Origin src_origin(src_url);
   url::Origin dest_origin(dest_url);
   auto* policy = ChildProcessSecurityPolicyImpl::GetInstance();
-  if (policy->IsIsolatedOrigin(src_origin) ||
-      policy->IsIsolatedOrigin(dest_origin))
-    return src_origin == dest_origin;
+  url::Origin src_isolated_origin;
+  url::Origin dest_isolated_origin;
+  bool src_origin_is_isolated =
+      policy->TryGetMostSpecificMatchForIsolatedOrigin(src_origin,
+                                                       &src_isolated_origin);
+  bool dest_origin_is_isolated =
+      policy->TryGetMostSpecificMatchForIsolatedOrigin(dest_origin,
+                                                       &dest_isolated_origin);
+  if (src_origin_is_isolated || dest_origin_is_isolated) {
+    // Compare most specific matching origins to ensure that a subdomain of an
+    // isolated origin (e.g., https://subdomain.isolated.foo.com) also matches
+    // the isolated origin's site URL (e.g., https://isolated.foo.com).
+    return src_isolated_origin == dest_isolated_origin;
+  }
 
   // If the schemes differ, they aren't part of the same site.
   if (src_origin.scheme() != dest_origin.scheme())
@@ -336,10 +348,14 @@ GURL SiteInstance::GetSiteForURL(BrowserContext* browser_context,
   GURL url = SiteInstanceImpl::GetEffectiveURL(browser_context, real_url);
   url::Origin origin(url);
 
-  // Isolated origins should use the full origin as their site URL.
+  // Isolated origins should use the full origin as their site URL. A subdomain
+  // of an isolated origin should also use that isolated origin's site URL.
   auto* policy = ChildProcessSecurityPolicyImpl::GetInstance();
-  if (policy->IsIsolatedOrigin(origin))
-    return origin.GetURL();
+  url::Origin isolated_origin;
+  if (policy->TryGetMostSpecificMatchForIsolatedOrigin(url::Origin(real_url),
+                                                       &isolated_origin)) {
+    return isolated_origin.GetURL();
+  }
 
   // If the url has a host, then determine the site.
   if (!origin.host().empty()) {