[SECURITY-3430]

src/main/java/hudson/remoting/Channel.java

@@ -52,11 +52,14 @@
 import java.net.URL;
 import java.nio.channels.ClosedChannelException;
 import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.Date;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Set;
 import java.util.WeakHashMap;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
@@ -925,15 +928,51 @@ public boolean preloadJar(Callable<?,?> classLoaderRef, Class<?>... classesInJar
         return preloadJar(UserRequest.getClassLoader(classLoaderRef), classesInJar);
     }
 
+    @SuppressFBWarnings(
+            value = "DMI_COLLECTION_OF_URLS",
+            justification = "All URLs point to local files, so no DNS lookup.")
     public boolean preloadJar(ClassLoader local, Class<?>... classesInJar) throws IOException, InterruptedException {
-        URL[] jars = new URL[classesInJar.length];
-        for (int i = 0; i < classesInJar.length; i++)
-            jars[i] = Which.jarFile(classesInJar[i]).toURI().toURL();
-        return call(new PreloadJarTask(jars, local));
+        Set<URL> jarSet = new HashSet<>();
+        for (Class<?> clazz : classesInJar) {
+            jarSet.add(Which.jarFile(clazz).toURI().toURL());
+        }
+        URL[] jars = jarSet.toArray(new URL[0]);
+        return preloadJar(local, jars);
     }
 
+    @SuppressFBWarnings(value = "URLCONNECTION_SSRF_FD", justification = "Callers are privileged controller-side code.")
     public boolean preloadJar(ClassLoader local, URL... jars) throws IOException, InterruptedException {
-        return call(new PreloadJarTask(jars,local));
+        byte[][] contents = new byte[jars.length][0];
+
+        List<URL> jarList = Arrays.asList(jars);
+        for (int i = 0; i < jarList.size(); i++) {
+            final URL url = jarList.get(i);
+            jars[i] = url;
+            contents[i] = Util.readFully(url.openStream());
+        }
+        try {
+            return call(new PreloadJarTask2(jars, contents, local));
+        } catch (IOException ex) {
+            if (ex.getCause() instanceof IllegalAccessError) {
+                logger.log(
+                        Level.FINE,
+                        ex,
+                        () -> "Failed to call PreloadJarTask2 on " + this + ", retrying with PreloadJarTask");
+                // When the agent is running an outdated version of remoting, we cannot access nonpublic classes in the
+                // same package, as PreloadJarTask2 would be loaded from the controller, and hence a different module/
+                // classloader, than the rest of remoting. As a result PreloadJarTask2 will throw IllegalAccessError:
+                //
+                // java.lang.IllegalAccessError: failed to access class hudson.remoting.RemoteClassLoader from class
+                // hudson.remoting.PreloadJarTask2 (hudson.remoting.RemoteClassLoader is in unnamed module of loader
+                // 'app'; hudson.remoting.PreloadJarTask2 is in unnamed module of loader 'Jenkins v${project.version}'
+                // @795f104a)
+                //
+                // Identify this error here and fall back to PreloadJarTask, relying on the restrictive controller-side
+                // implementation of IClassLoader#fetchJar.
+                return call(new PreloadJarTask(jars, local));
+            }
+            throw ex;
+        }
     }
 
     /**

src/main/java/hudson/remoting/JarURLValidator.java

@@ -0,0 +1,15 @@
+package hudson.remoting;
+
+import java.io.IOException;
+import java.net.URL;
+
+/**
+ * Validate a URL attempted to be read by the remote end (agent side).
+ *
+ * @deprecated Do not use, intended as a temporary workaround only.
+ */
+// TODO Remove once we no longer require compatibility with remoting before 2024-08.
+@Deprecated
+public interface JarURLValidator {
+    void validate(URL url) throws IOException;
+}

src/main/java/hudson/remoting/PreloadJarTask.java

@@ -34,8 +34,10 @@
  * {@link Callable} used to deliver a jar file to {@link RemoteClassLoader}.
  *
  * @author Kohsuke Kawaguchi
+ * @deprecated Retained for compatibility with pre-2024-08 remoting only (see {@link Channel#preloadJar(ClassLoader, java.net.URL...)}), use {@link hudson.remoting.PreloadJarTask2}.
  */
-final class PreloadJarTask implements DelegatingCallable<Boolean,IOException> {
+@Deprecated
+final class PreloadJarTask implements DelegatingCallable<Boolean, IOException> {
     /**
      * Jar file to be preloaded.
      */

src/main/java/hudson/remoting/PreloadJarTask2.java

@@ -0,0 +1,100 @@
+/*
+ * The MIT License
+ *
+ * Copyright (c) 2004-2009, Sun Microsystems, Inc., Kohsuke Kawaguchi
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package hudson.remoting;
+
+import edu.umd.cs.findbugs.annotations.CheckForNull;
+import java.io.IOException;
+import java.net.URL;
+import org.jenkinsci.remoting.Role;
+import org.jenkinsci.remoting.RoleChecker;
+
+/**
+ * {@link Callable} used to deliver a jar file to {@link RemoteClassLoader}.
+ * <p>
+ *  This replaces {@link hudson.remoting.PreloadJarTask} and delivers the jar contents as part of the Callable rather
+ *  than needing to call {@link hudson.remoting.RemoteClassLoader#prefetch(java.net.URL)}.
+ * </p>
+ * @since TODO 2024-08
+ */
+final class PreloadJarTask2 implements DelegatingCallable<Boolean, IOException> {
+    /**
+     * Jar file to be preloaded.
+     */
+    private final URL[] jars;
+
+    private final byte[][] contents;
+
+    // TODO: This implementation exists starting from
+    //  https://github.com/jenkinsci/remoting/commit/f3d0a81fdf46a10c3c6193faf252efaeaee98823
+    //  Since this time nothing has blown up, but it still seems to be suspicious.
+    //  The solution for null classloaders is available in RemoteDiagnostics.Script#call() in the Jenkins core codebase
+    @CheckForNull
+    private transient ClassLoader target = null;
+
+    PreloadJarTask2(URL[] jars, byte[][] contents, @CheckForNull ClassLoader target) {
+        if (jars.length != contents.length) {
+            throw new IllegalArgumentException("Got " + jars.length + " jars and " + contents.length + " contents");
+        }
+        this.jars = jars;
+        this.contents = contents;
+        this.target = target;
+    }
+
+    @Override
+    public ClassLoader getClassLoader() {
+        return target;
+    }
+
+    @Override
+    public Boolean call() throws IOException {
+        ClassLoader cl = Thread.currentThread().getContextClassLoader();
+
+        try {
+            if (!(cl instanceof RemoteClassLoader)) {
+                return false;
+            }
+            final RemoteClassLoader rcl = (RemoteClassLoader) cl;
+
+            boolean r = false;
+            for (int i = 0; i < jars.length; i++) {
+                r |= rcl.prefetch(jars[i], contents[i]);
+            }
+            return r;
+        } catch (IllegalAccessError iae) {
+            // Catch the IAE instead of letting it be wrapped by remoting to suppress warnings logged on the agent-side
+            throw new IOException(iae);
+        }
+    }
+
+    /**
+     * This task is only useful in the context that allows remote classloading, and by that point
+     * any access control check is pointless. So just declare the worst possible role.
+     */
+    @Override
+    public void checkRoles(RoleChecker checker) throws SecurityException {
+        checker.check(this, Role.UNKNOWN);
+    }
+
+    private static final long serialVersionUID = -773448303394727271L;
+}

src/main/java/hudson/remoting/RemoteClassLoader.java

@@ -669,8 +669,12 @@ public static void deleteDirectoryOnExit(File dir) {
      * @param jar Jar to be prefetched. Note that this file is an file on the other end,
      *            and doesn't point to anything meaningful locally.
      * @return true if the prefetch happened. false if the jar is already prefetched.
+     * @deprecated Only left in for compatibility with pre-2024-08 remoting. Use {@link #prefetch(java.net.URL, byte[])} instead.
      * @see Channel#preloadJar(Callable, Class[])
+     * @see hudson.remoting.PreloadJarTask
+     * @see hudson.remoting.PreloadJarTask2
      */
+    @Deprecated
     /*package*/ boolean prefetch(URL jar) throws IOException {
         synchronized (prefetchedJars) {
             if (prefetchedJars.contains(jar)) {
@@ -686,6 +690,31 @@ public static void deleteDirectoryOnExit(File dir) {
         }
     }
 
+    /**
+     * Prefetches the specified jar with the specified content into this classloader.
+     * @param jar Jar to be prefetched. Note that this file is an file on the other end,
+     *            and doesn't point to anything meaningful locally.
+     * @param content the jar content
+     * @return true if the prefetch happened. false if the jar is already prefetched.
+     * @see Channel#preloadJar(Callable, Class[])
+     * @see hudson.remoting.PreloadJarTask2
+     * @since TODO 2024-08
+     */
+    /*package*/ boolean prefetch(URL jar, byte[] content) throws IOException {
+        synchronized (prefetchedJars) {
+            if (prefetchedJars.contains(jar)) {
+                return false;
+            }
+
+            String p = jar.getPath().replace('\\', '/');
+            p = Util.getBaseName(p);
+            File localJar = Util.makeResource(p, content);
+            addURL(localJar.toURI().toURL());
+            prefetchedJars.add(jar);
+            return true;
+        }
+    }
+
     /**
      * Receiver-side of {@link ClassFile2} uses this to remember the prefetch information.
      */
@@ -828,6 +857,7 @@ public static class ClassFile2 extends ResourceFile {
      * Remoting interface.
      */
     public interface IClassLoader {
+        @Deprecated
         byte[] fetchJar(URL url) throws IOException;
 
         /**
@@ -945,8 +975,27 @@ public ClassLoaderProxy(@NonNull ClassLoader cl, Channel channel) {
         }
 
         @Override
-        @SuppressFBWarnings(value = "URLCONNECTION_SSRF_FD", justification = "This is only used for managing the jar cache as files.")
+        @SuppressFBWarnings(
+                value = "URLCONNECTION_SSRF_FD",
+                justification = "URL validation is being done through JarURLValidator")
         public byte[] fetchJar(URL url) throws IOException {
+            final Object o = channel.getProperty(JarURLValidator.class);
+            if (o == null) {
+                final boolean disabled = Boolean.getBoolean(Channel.class.getName() + ".DISABLE_JAR_URL_VALIDATOR");
+                LOGGER.log(Level.FINE, "Default behavior for URL: " + url + " with disabled flag: " + disabled);
+                if (!disabled) {
+                    throw new IOException(
+                            "No hudson.remoting.JarURLValidator has been set for this channel, so all #fetchJar calls are rejected."
+                                    + " This is likely a bug in Jenkins."
+                                    + " As a workaround, try updating the agent.jar file.");
+                }
+            } else {
+                if (o instanceof JarURLValidator) {
+                    ((JarURLValidator) o).validate(url);
+                } else {
+                    throw new IOException("Unexpected channel property hudson.remoting.JarURLValidator value: " + o);
+                }
+            }
             return readFully(url.openStream());
         }