Invalid JSON request body caused endless loop (#26680)

Request bodys that only consists of a String value can lead to endless loops in the
parser of several rest requests like e.g. `_count`. Up to 5.2 this seems to have been caught
in the logic guessing the content type of the request, but since then it causes the node to
block. This change introduces checks for receiving a valid xContent object before starting the
parsing in RestActions#parseTopLevelQueryBuilder().

Closes #26083

core/src/main/java/org/elasticsearch/index/query/QueryParseContext.java

@@ -66,6 +66,15 @@ public boolean isDeprecatedSetting(String setting) {
     public QueryBuilder parseTopLevelQueryBuilder() {
         try {
             QueryBuilder queryBuilder = null;
+            XContentParser.Token first = parser.nextToken();
+            if (first == null) {
+                return null;
+            } else if (first != XContentParser.Token.START_OBJECT) {
+                throw new ParsingException(
+                    parser.getTokenLocation(), "Expected [" + XContentParser.Token.START_OBJECT +
+                    "] but found [" + first + "]", parser.getTokenLocation()
+                );
+            }
             for (XContentParser.Token token = parser.nextToken(); token != XContentParser.Token.END_OBJECT; token = parser.nextToken()) {
                 if (token == XContentParser.Token.FIELD_NAME) {
                     String fieldName = parser.currentName();
@@ -110,7 +119,7 @@ public Optional<QueryBuilder> parseInnerQueryBuilder() throws IOException {
         Optional<QueryBuilder> result;
         try {
             @SuppressWarnings("unchecked")
-            Optional<QueryBuilder> resultCast = (Optional<QueryBuilder>) parser.namedObject(Optional.class, queryName, this); 
+            Optional<QueryBuilder> resultCast = (Optional<QueryBuilder>) parser.namedObject(Optional.class, queryName, this);
             result = resultCast;
         } catch (UnknownNamedObjectException e) {
             // Preserve the error message from 5.0 until we have a compellingly better message so we don't break BWC.

core/src/test/java/org/elasticsearch/rest/action/RestActionsTests.java

@@ -0,0 +1,106 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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 org.elasticsearch.rest.action;
+
+import com.fasterxml.jackson.core.io.JsonEOFException;
+import java.util.Arrays;
+import org.elasticsearch.common.ParsingException;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.NamedXContentRegistry;
+import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.common.xcontent.json.JsonXContent;
+import org.elasticsearch.index.query.MatchQueryBuilder;
+import org.elasticsearch.index.query.QueryBuilder;
+import org.elasticsearch.search.SearchModule;
+import org.elasticsearch.test.ESTestCase;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+
+import java.io.IOException;
+
+import static java.util.Collections.emptyList;
+
+public class RestActionsTests extends ESTestCase {
+
+    private static NamedXContentRegistry xContentRegistry;
+
+    @BeforeClass
+    public static void init() {
+        xContentRegistry = new NamedXContentRegistry(new SearchModule(Settings.EMPTY, false, emptyList()).getNamedXContents());
+    }
+
+    @AfterClass
+    public static void cleanup() {
+        xContentRegistry = null;
+    }
+
+    public void testParseTopLevelBuilder() throws IOException {
+        QueryBuilder query = new MatchQueryBuilder("foo", "bar");
+        String requestBody = "{ \"query\" : " + query.toString() + "}";
+        try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+            QueryBuilder actual = RestActions.getQueryContent(parser);
+            assertEquals(query, actual);
+        }
+    }
+
+    public void testParseTopLevelBuilderEmptyObject() throws IOException {
+        for (String requestBody : Arrays.asList("{}", "")) {
+            try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+                QueryBuilder query = RestActions.getQueryContent(parser);
+                assertNull(query);
+            }
+        }
+    }
+
+    public void testParseTopLevelBuilderMalformedJson() throws IOException {
+        for (String requestBody : Arrays.asList("\"\"", "\"someString\"", "\"{\"")) {
+            try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+                ParsingException exception =
+                    expectThrows(ParsingException.class, () -> RestActions.getQueryContent(parser));
+                assertEquals("Expected [START_OBJECT] but found [VALUE_STRING]", exception.getMessage());
+            }
+        }
+    }
+
+    public void testParseTopLevelBuilderIncompleteJson() throws IOException {
+        for (String requestBody : Arrays.asList("{", "{ \"query\" :")) {
+            try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+                ParsingException exception =
+                    expectThrows(ParsingException.class, () -> RestActions.getQueryContent(parser));
+                assertEquals("Failed to parse", exception.getMessage());
+                assertEquals(JsonEOFException.class, exception.getRootCause().getClass());
+            }
+        }
+    }
+
+    public void testParseTopLevelBuilderUnknownParameter() throws IOException {
+        String requestBody = "{ \"foo\" : \"bar\"}";
+        try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+            ParsingException exception = expectThrows(ParsingException.class, () -> RestActions.getQueryContent(parser));
+            assertEquals("request does not support [foo]", exception.getMessage());
+        }
+    }
+
+    @Override
+    protected NamedXContentRegistry xContentRegistry() {
+        return xContentRegistry;
+    }
+
+}