SshDaemon: Use channel id tracking for unknown reference handler

Extend the functionality of the `DefaultUnknownChannelReferenceHandler`
by tracking the initialized channels identifiers and being lenient only
if command is received for a channel that was initialized in the past.

The code is borrowed from the Apache SSHD Mina project's sshd-contrib
sub-project, moved to the com.google.gerrit.sshd package and run
through google-java-format utility. Slf4j logger was also replaced with
flogger. See this issue: [1] and this CL for more details: [2].

For more safety, protect this experimental feature behind undocumented
configuration option, but enable this option per default.

[1] https://issues.apache.org/jira/browse/SSHD-942
[2] 11b33dee37

Feature: Issue 11572
Change-Id: Ib41c94fc43aca7c4916d40128f03b643cee66d9a

java/com/google/gerrit/sshd/ChannelIdTrackingUnknownChannelReferenceHandler.java

@@ -0,0 +1,90 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/**
+ * This file is based on sshd-contrib Apache SSHD Mina project. Original commit:
+ * https://github.com/apache/mina-sshd/commit/11b33dee37b5b9c71a40a8a98a42007e3687131e
+ */
+package com.google.gerrit.sshd;
+
+import com.google.common.flogger.FluentLogger;
+import java.io.IOException;
+import org.apache.sshd.common.AttributeRepository.AttributeKey;
+import org.apache.sshd.common.SshConstants;
+import org.apache.sshd.common.channel.Channel;
+import org.apache.sshd.common.channel.ChannelListener;
+import org.apache.sshd.common.channel.exception.SshChannelNotFoundException;
+import org.apache.sshd.common.session.ConnectionService;
+import org.apache.sshd.common.session.Session;
+import org.apache.sshd.common.session.helpers.DefaultUnknownChannelReferenceHandler;
+import org.apache.sshd.common.util.buffer.Buffer;
+
+/**
+ * Makes sure that the referenced "unknown" channel identifier is one that was assigned in
+ * the past. <B>Note:</B> it relies on the fact that the default {@code ConnectionService}
+ * implementation assigns channels identifiers in ascending order.
+ *
+ * @author <a href="mailto:dev@mina.apache.org">Apache MINA SSHD Project</a>
+ */
+public class ChannelIdTrackingUnknownChannelReferenceHandler
+    extends DefaultUnknownChannelReferenceHandler implements ChannelListener {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+  public static final AttributeKey<Integer> LAST_CHANNEL_ID_KEY = new AttributeKey<>();
+
+  public static final ChannelIdTrackingUnknownChannelReferenceHandler TRACKER =
+      new ChannelIdTrackingUnknownChannelReferenceHandler();
+
+  public ChannelIdTrackingUnknownChannelReferenceHandler() {
+    super();
+  }
+
+  @Override
+  public void channelInitialized(Channel channel) {
+    int channelId = channel.getId();
+    Session session = channel.getSession();
+    Integer lastTracked = session.setAttribute(LAST_CHANNEL_ID_KEY, channelId);
+    logger.atFine().log(
+        "channelInitialized(%s) updated last tracked channel ID %s => %s",
+        channel, lastTracked, channelId);
+  }
+
+  @Override
+  public Channel handleUnknownChannelCommand(
+      ConnectionService service, byte cmd, int channelId, Buffer buffer) throws IOException {
+    Session session = service.getSession();
+    Integer lastTracked = session.getAttribute(LAST_CHANNEL_ID_KEY);
+    if ((lastTracked != null) && (channelId <= lastTracked.intValue())) {
+      // Use TRACE level in order to avoid messages flooding
+      logger.atFinest().log(
+          "handleUnknownChannelCommand(%s) apply default handling for %s on channel=%s (lastTracked=%s)",
+          session, SshConstants.getCommandMessageName(cmd), channelId, lastTracked);
+      return super.handleUnknownChannelCommand(service, cmd, channelId, buffer);
+    }
+
+    throw new SshChannelNotFoundException(
+        channelId,
+        "Received "
+            + SshConstants.getCommandMessageName(cmd)
+            + " on unassigned channel "
+            + channelId
+            + " (last assigned="
+            + lastTracked
+            + ")");
+  }
+}

java/com/google/gerrit/sshd/SshDaemon.java

@@ -209,6 +209,7 @@ public class SshDaemon extends SshServer implements SshInfo, LifecycleListener {
     final boolean enableCompression = cfg.getBoolean("sshd", "enableCompression", false);
 
     SshSessionBackend backend = cfg.getEnum("sshd", null, "backend", SshSessionBackend.NIO2);
+    boolean channelIdTracking = cfg.getBoolean("sshd", "enableChannelIdTracking", true);
 
     System.setProperty(
         IoServiceFactoryFactory.class.getName(),
@@ -222,7 +223,7 @@ public class SshDaemon extends SshServer implements SshInfo, LifecycleListener {
     initMacs(cfg);
     initSignatures();
     initChannels();
-    initUnknownChannelReferenceHandler();
+    initUnknownChannelReferenceHandler(channelIdTracking);
     initForwarding();
     initFileSystemFactory();
     initSubsystems();
@@ -653,8 +654,11 @@ public class SshDaemon extends SshServer implements SshInfo, LifecycleListener {
     setChannelFactories(ServerBuilder.DEFAULT_CHANNEL_FACTORIES);
   }
 
-  private void initUnknownChannelReferenceHandler() {
+  private void initUnknownChannelReferenceHandler(boolean enableChannelIdTracking) {
-    setUnknownChannelReferenceHandler(DefaultUnknownChannelReferenceHandler.INSTANCE);
+    setUnknownChannelReferenceHandler(
+        enableChannelIdTracking
+            ? ChannelIdTrackingUnknownChannelReferenceHandler.TRACKER
+            : DefaultUnknownChannelReferenceHandler.INSTANCE);
   }
 
   private void initSubsystems() {