Allow system privilege to execute proxied actions (#37508)

Currently all proxied actions are denied for the `SystemPrivilege`.
Unfortunately, there are use cases (CCR) where we would like to proxy
actions to a remote node that are normally performed by the
system context. This commit allows the system context to perform
proxy actions if they are actions that the system context is normally
allowed to execute.

Author: Tim-Brooks

server/src/main/java/org/elasticsearch/transport/TransportActionProxy.java

@@ -175,6 +175,14 @@ public static TransportRequest unwrapRequest(TransportRequest request) {
         return request;
     }
 
+    /**
+     * Unwraps a proxy action and returns the underlying action
+     */
+    public static String unwrapAction(String action) {
+        assert isProxyAction(action) : "Attempted to unwrap non-proxy action: " + action;
+        return action.substring(PROXY_ACTION_PREFIX.length());
+    }
+
     /**
      * Returns <code>true</code> iff the given action is a proxy action
      */

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/privilege/SystemPrivilege.java

@@ -5,6 +5,7 @@
  */
 package org.elasticsearch.xpack.core.security.authz.privilege;
 
+import org.elasticsearch.transport.TransportActionProxy;
 import org.elasticsearch.xpack.core.security.support.Automatons;
 
 import java.util.Collections;
@@ -14,19 +15,27 @@ public final class SystemPrivilege extends Privilege {
 
     public static SystemPrivilege INSTANCE = new SystemPrivilege();
 
-    private static final Predicate<String> PREDICATE = Automatons.predicate(Automatons.
-            minusAndMinimize(Automatons.patterns(
-            "internal:*",
-            "indices:monitor/*", // added for monitoring
-            "cluster:monitor/*",  // added for monitoring
-            "cluster:admin/bootstrap/*", // for the bootstrap service
-            "cluster:admin/reroute", // added for DiskThresholdDecider.DiskListener
-            "indices:admin/mapping/put", // needed for recovery and shrink api
-            "indices:admin/template/put", // needed for the TemplateUpgradeService
-            "indices:admin/template/delete", // needed for the TemplateUpgradeService
-            "indices:admin/seq_no/global_checkpoint_sync*", // needed for global checkpoint syncs
-            "indices:admin/settings/update" // needed for DiskThresholdMonitor.markIndicesReadOnly
-    ), Automatons.patterns("internal:transport/proxy/*"))); // no proxy actions for system user!
+    private static final Predicate<String> ALLOWED_ACTIONS = Automatons.predicate(
+        "internal:*",
+        "indices:monitor/*", // added for monitoring
+        "cluster:monitor/*",  // added for monitoring
+        "cluster:admin/bootstrap/*", // for the bootstrap service
+        "cluster:admin/reroute", // added for DiskThresholdDecider.DiskListener
+        "indices:admin/mapping/put", // needed for recovery and shrink api
+        "indices:admin/template/put", // needed for the TemplateUpgradeService
+        "indices:admin/template/delete", // needed for the TemplateUpgradeService
+        "indices:admin/seq_no/global_checkpoint_sync*", // needed for global checkpoint syncs
+        "indices:admin/settings/update" // needed for DiskThresholdMonitor.markIndicesReadOnly
+    );
+
+    private static final Predicate<String> PREDICATE = (action) -> {
+        // Only allow a proxy action if the underlying action is allowed
+        if (TransportActionProxy.isProxyAction(action)) {
+            return ALLOWED_ACTIONS.test(TransportActionProxy.unwrapAction(action));
+        } else {
+            return ALLOWED_ACTIONS.test(action);
+        }
+    };
 
     private SystemPrivilege() {
         super(Collections.singleton("internal"));

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/security/authz/privilege/PrivilegeTests.java

@@ -123,6 +123,7 @@ public void testSystem() throws Exception {
         assertThat(predicate.test("indices:admin/mapping/put"), is(true));
         assertThat(predicate.test("indices:admin/mapping/whatever"), is(false));
         assertThat(predicate.test("internal:transport/proxy/indices:data/read/query"), is(false));
+        assertThat(predicate.test("internal:transport/proxy/indices:monitor/whatever"), is(true));
         assertThat(predicate.test("indices:admin/seq_no/global_checkpoint_sync"), is(true));
         assertThat(predicate.test("indices:admin/seq_no/global_checkpoint_sync[p]"), is(true));
         assertThat(predicate.test("indices:admin/seq_no/global_checkpoint_sync[r]"), is(true));