[JENKINS-41684] Ensure that PluginManager.dynamicLoad runs as SYSTEM (#2732)

* [FIXED JENKINS-41684] Ensure that PluginManager.dynamicLoad runs as SYSTEM.
Test plugin source:
package test;
import hudson.Plugin;
import jenkins.model.Jenkins;
public class ThePlugin extends Plugin {
    @Override
    public void postInitialize() throws Exception {
        Jenkins.getInstance().checkPermission(Jenkins.ADMINISTER);
    }
}

* @daniel-beck wants this all reindented.

core/src/main/java/hudson/PluginManager.java

@@ -819,6 +819,7 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
      */
     @Restricted(NoExternalUse.class)
     public void dynamicLoad(File arc, boolean removeExisting) throws IOException, InterruptedException, RestartRequiredException {
+        try (ACLContext context = ACL.as(ACL.SYSTEM)) {
             LOGGER.info("Attempting to dynamic load "+arc);
             PluginWrapper p = null;
             String sn;
@@ -914,6 +915,7 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
 
             LOGGER.info("Plugin " + p.getShortName()+":"+p.getVersion() + " dynamically installed");
         }
+    }
 
     @Restricted(NoExternalUse.class)
     public synchronized void resolveDependantPlugins() {

test/src/test/java/hudson/PluginManagerTest.java

@@ -30,7 +30,10 @@ import hudson.model.Hudson;
 import hudson.model.UpdateCenter;
 import hudson.model.UpdateCenter.UpdateCenterJob;
 import hudson.model.UpdateSite;
+import hudson.model.User;
 import hudson.scm.SubversionSCM;
+import hudson.security.ACL;
+import hudson.security.ACLContext;
 import hudson.util.FormValidation;
 import hudson.util.PersistedList;
 import java.io.File;
@@ -53,6 +56,7 @@ import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
 import org.jvnet.hudson.test.Issue;
 import org.jvnet.hudson.test.JenkinsRule;
+import org.jvnet.hudson.test.MockAuthorizationStrategy;
 import org.jvnet.hudson.test.Url;
 import org.jvnet.hudson.test.recipes.WithPlugin;
 import org.jvnet.hudson.test.recipes.WithPluginManager;
@@ -444,6 +448,16 @@ public class PluginManagerTest {
         assertTrue(pluginInfo.getString("dependencies") != null);
     }
 
+    @Issue("JENKINS-41684")
+    @Test
+    public void requireSystemDuringLoad() throws Exception {
+        r.jenkins.setSecurityRealm(r.createDummySecurityRealm());
+        r.jenkins.setAuthorizationStrategy(new MockAuthorizationStrategy());
+        try (ACLContext context = ACL.as(User.get("underprivileged").impersonate())) {
+            dynamicLoad("require-system-during-load.hpi");
+        }
+    }
+
     private void dynamicLoad(String plugin) throws IOException, InterruptedException, RestartRequiredException {
         PluginManagerUtil.dynamicLoad(plugin, r.jenkins);
     }