netty: Limit number of frames client can cause server to enqueue

Http2ControlFrameLimitEncoder is from Netty. It is copied here as a
temporary measure until we upgrade to the version of Netty that includes
the class.

See CVE-2019-9515

netty/build.gradle

@@ -40,3 +40,7 @@ jmh {
     // https://github.com/melix/jmh-gradle-plugin/issues/97#issuecomment-316664026
     includeTests = true
 }
+
+checkstyleMain {
+    source = source.minus(fileTree(dir: "src/main", include: "**/Http2ControlFrameLimitEncoder.java"))
+}

netty/src/main/java/io/grpc/netty/Http2ControlFrameLimitEncoder.java

@@ -0,0 +1,118 @@
+/*
+ * Copyright 2019 The Netty Project
+ *
+ * The Netty Project 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.
+ */
+package io.grpc.netty;
+
+import io.netty.channel.ChannelFuture;
+import io.netty.channel.ChannelFutureListener;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.ChannelPromise;
+import io.netty.handler.codec.http2.DecoratingHttp2ConnectionEncoder;
+import io.netty.handler.codec.http2.Http2ConnectionEncoder;
+import io.netty.handler.codec.http2.Http2Error;
+import io.netty.handler.codec.http2.Http2Exception;
+import io.netty.handler.codec.http2.Http2LifecycleManager;
+import io.netty.util.internal.ObjectUtil;
+import io.netty.util.internal.logging.InternalLogger;
+import io.netty.util.internal.logging.InternalLoggerFactory;
+
+/**
+ * {@link DecoratingHttp2ConnectionEncoder} which guards against a remote peer that will trigger a massive amount
+ * of control frames but will not consume our responses to these.
+ * This encoder will tear-down the connection once we reached the configured limit to reduce the risk of DDOS.
+ */
+final class Http2ControlFrameLimitEncoder extends DecoratingHttp2ConnectionEncoder {
+    private static final InternalLogger logger = InternalLoggerFactory.getInstance(Http2ControlFrameLimitEncoder.class);
+
+    private final int maxOutstandingControlFrames;
+    private final ChannelFutureListener outstandingControlFramesListener = new ChannelFutureListener() {
+        @Override
+        public void operationComplete(ChannelFuture future) {
+            outstandingControlFrames--;
+        }
+    };
+    private Http2LifecycleManager lifecycleManager;
+    private int outstandingControlFrames;
+    private boolean limitReached;
+
+    Http2ControlFrameLimitEncoder(Http2ConnectionEncoder delegate, int maxOutstandingControlFrames) {
+        super(delegate);
+        this.maxOutstandingControlFrames = ObjectUtil.checkPositive(maxOutstandingControlFrames,
+                "maxOutstandingControlFrames");
+    }
+
+    @Override
+    public void lifecycleManager(Http2LifecycleManager lifecycleManager) {
+        this.lifecycleManager = lifecycleManager;
+        super.lifecycleManager(lifecycleManager);
+    }
+
+    @Override
+    public ChannelFuture writeSettingsAck(ChannelHandlerContext ctx, ChannelPromise promise) {
+        ChannelPromise newPromise = handleOutstandingControlFrames(ctx, promise);
+        if (newPromise == null) {
+            return promise;
+        }
+        return super.writeSettingsAck(ctx, newPromise);
+    }
+
+    @Override
+    public ChannelFuture writePing(ChannelHandlerContext ctx, boolean ack, long data, ChannelPromise promise) {
+        // Only apply the limit to ping acks.
+        if (ack) {
+            ChannelPromise newPromise = handleOutstandingControlFrames(ctx, promise);
+            if (newPromise == null) {
+                return promise;
+            }
+            return super.writePing(ctx, ack, data, newPromise);
+        }
+        return super.writePing(ctx, ack, data, promise);
+    }
+
+    @Override
+    public ChannelFuture writeRstStream(
+            ChannelHandlerContext ctx, int streamId, long errorCode, ChannelPromise promise) {
+        ChannelPromise newPromise = handleOutstandingControlFrames(ctx, promise);
+        if (newPromise == null) {
+            return promise;
+        }
+        return super.writeRstStream(ctx, streamId, errorCode, newPromise);
+    }
+
+    private ChannelPromise handleOutstandingControlFrames(ChannelHandlerContext ctx, ChannelPromise promise) {
+        if (!limitReached) {
+            if (outstandingControlFrames == maxOutstandingControlFrames) {
+                // Let's try to flush once as we may be able to flush some of the control frames.
+                ctx.flush();
+            }
+            if (outstandingControlFrames == maxOutstandingControlFrames) {
+                limitReached = true;
+                Http2Exception exception = Http2Exception.connectionError(Http2Error.ENHANCE_YOUR_CALM,
+                        "Maximum number %d of outstanding control frames reached", maxOutstandingControlFrames);
+                logger.info("Maximum number {} of outstanding control frames reached. Closing channel {}",
+                        maxOutstandingControlFrames, ctx.channel(), exception);
+
+                // First notify the Http2LifecycleManager and then close the connection.
+                lifecycleManager.onError(ctx, true, exception);
+                ctx.close();
+            }
+            outstandingControlFrames++;
+
+            // We did not reach the limit yet, add the listener to decrement the number of outstanding control frames
+            // once the promise was completed
+            return promise.unvoid().addListener(outstandingControlFramesListener);
+        }
+        return promise;
+    }
+}

netty/src/main/java/io/grpc/netty/NettyServerHandler.java

@@ -214,6 +214,7 @@ class NettyServerHandler extends AbstractNettyHandler {
         new DefaultHttp2LocalFlowController(connection, DEFAULT_WINDOW_UPDATE_RATIO, true));
     frameWriter = new WriteMonitoringFrameWriter(frameWriter, keepAliveEnforcer);
     Http2ConnectionEncoder encoder = new DefaultHttp2ConnectionEncoder(connection, frameWriter);
+    encoder = new Http2ControlFrameLimitEncoder(encoder, 10000);
     Http2ConnectionDecoder decoder = new DefaultHttp2ConnectionDecoder(connection, encoder,
         frameReader);