Follow symlinks in Docker entrypoint (#51102)

Backport of #50927.

Closes #49653. When using _FILE environment variables to supply values
to Elasticsearch, following symlinks when checking that file permissions
are secure.

distribution/src/bin/elasticsearch-env-from-file

@@ -24,10 +24,14 @@ for VAR_NAME_FILE in $(env | cut -f1 -d= | grep '_FILE$'); do
       exit 1
     fi
 
-    FILE_PERMS="$(stat -c '%a' ${!VAR_NAME_FILE})"
-
-    if [[ "$FILE_PERMS" != "400" && "$FILE_PERMS" != 600 ]]; then
-        echo "ERROR: File ${!VAR_NAME_FILE} from $VAR_NAME_FILE must have file permissions 400 or 600, but actually has: $FILE_PERMS" >&2
+    FILE_PERMS="$(stat -L -c '%a' ${!VAR_NAME_FILE})"
+
+    if [[ "$FILE_PERMS" != "400" && "$FILE_PERMS" != "600" ]]; then
+        if [[ -h "${!VAR_NAME_FILE}" ]]; then
+            echo "ERROR: File $(readlink "${!VAR_NAME_FILE}") (target of symlink ${!VAR_NAME_FILE} from $VAR_NAME_FILE) must have file permissions 400 or 600, but actually has: $FILE_PERMS" >&2
+        else
+            echo "ERROR: File ${!VAR_NAME_FILE} from $VAR_NAME_FILE must have file permissions 400 or 600, but actually has: $FILE_PERMS" >&2
+        fi
         exit 1
     fi
 

qa/os/src/test/java/org/elasticsearch/packaging/test/DockerTests.java

@@ -38,6 +38,7 @@
 import static org.elasticsearch.packaging.util.Docker.waitForPathToExist;
 import static org.elasticsearch.packaging.util.FileMatcher.p600;
 import static org.elasticsearch.packaging.util.FileMatcher.p660;
+import static org.elasticsearch.packaging.util.FileMatcher.p775;
 import static org.elasticsearch.packaging.util.FileUtils.append;
 import static org.elasticsearch.packaging.util.FileUtils.getTempDir;
 import static org.elasticsearch.packaging.util.FileUtils.rm;
@@ -52,14 +53,15 @@
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.nullValue;
+import static org.junit.Assume.assumeFalse;
 import static org.junit.Assume.assumeTrue;
 
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
@@ -338,10 +340,48 @@ public void test081ConfigurePasswordThroughEnvironmentVariableFile() throws Exce
         assertThat("Expected server to require authentication", statusCode, equalTo(401));
     }
 
+    /**
+     * Check that when verifying the file permissions of _FILE environment variables, symlinks
+     * are followed.
+     */
+    public void test082SymlinksAreFollowedWithEnvironmentVariableFiles() throws Exception {
+        // Test relies on configuring security
+        assumeTrue(distribution.isDefault());
+        // Test relies on symlinks
+        assumeFalse(Platforms.WINDOWS);
+
+        final String xpackPassword = "hunter2";
+        final String passwordFilename = "password.txt";
+        final String symlinkFilename = "password_symlink";
+
+        // ELASTIC_PASSWORD_FILE
+        Files.write(tempDir.resolve(passwordFilename), (xpackPassword + "\n").getBytes(StandardCharsets.UTF_8));
+
+        // Link to the password file. We can't use an absolute path for the target, because
+        // it won't resolve inside the container.
+        Files.createSymbolicLink(tempDir.resolve(symlinkFilename), Paths.get(passwordFilename));
+
+        // Enable security so that we can test that the password has been used
+        Map<String, String> envVars = new HashMap<>();
+        envVars.put("ELASTIC_PASSWORD_FILE", "/run/secrets/" + symlinkFilename);
+        envVars.put("xpack.security.enabled", "true");
+
+        // File permissions need to be secured in order for the ES wrapper to accept
+        // them for populating env var values. The wrapper will resolve the symlink
+        // and check the target's permissions.
+        Files.setPosixFilePermissions(tempDir.resolve(passwordFilename), p600);
+
+        final Map<Path, Path> volumes = singletonMap(tempDir, Paths.get("/run/secrets"));
+
+        // Restart the container - this will check that Elasticsearch started correctly,
+        // and didn't fail to follow the symlink and check the file permissions
+        runContainer(distribution(), volumes, envVars);
+    }
+
     /**
      * Check that environment variables cannot be used with _FILE environment variables.
      */
-    public void test081CannotUseEnvVarsAndFiles() throws Exception {
+    public void test083CannotUseEnvVarsAndFiles() throws Exception {
         final String optionsFilename = "esJavaOpts.txt";
 
         // ES_JAVA_OPTS_FILE
@@ -369,7 +409,7 @@ public void test081CannotUseEnvVarsAndFiles() throws Exception {
      * Check that when populating environment variables by setting variables with the suffix "_FILE",
      * the files' permissions are checked.
      */
-    public void test082EnvironmentVariablesUsingFilesHaveCorrectPermissions() throws Exception {
+    public void test084EnvironmentVariablesUsingFilesHaveCorrectPermissions() throws Exception {
         final String optionsFilename = "esJavaOpts.txt";
 
         // ES_JAVA_OPTS_FILE
@@ -393,25 +433,68 @@ public void test082EnvironmentVariablesUsingFilesHaveCorrectPermissions() throws
         );
     }
 
+    /**
+     * Check that when verifying the file permissions of _FILE environment variables, symlinks
+     * are followed, and that invalid target permissions are detected.
+     */
+    public void test085SymlinkToFileWithInvalidPermissionsIsRejected() throws Exception {
+        // Test relies on configuring security
+        assumeTrue(distribution.isDefault());
+        // Test relies on symlinks
+        assumeFalse(Platforms.WINDOWS);
+
+        final String xpackPassword = "hunter2";
+        final String passwordFilename = "password.txt";
+        final String symlinkFilename = "password_symlink";
+
+        // ELASTIC_PASSWORD_FILE
+        Files.write(tempDir.resolve(passwordFilename), (xpackPassword + "\n").getBytes(StandardCharsets.UTF_8));
+
+        // Link to the password file. We can't use an absolute path for the target, because
+        // it won't resolve inside the container.
+        Files.createSymbolicLink(tempDir.resolve(symlinkFilename), Paths.get(passwordFilename));
+
+        // Enable security so that we can test that the password has been used
+        Map<String, String> envVars = new HashMap<>();
+        envVars.put("ELASTIC_PASSWORD_FILE", "/run/secrets/" + symlinkFilename);
+        envVars.put("xpack.security.enabled", "true");
+
+        // Set invalid permissions on the file that the symlink targets
+        Files.setPosixFilePermissions(tempDir.resolve(passwordFilename), p775);
+
+        final Map<Path, Path> volumes = singletonMap(tempDir, Paths.get("/run/secrets"));
+
+        // Restart the container
+        final Result dockerLogs = runContainerExpectingFailure(distribution(), volumes, envVars);
+
+        assertThat(
+            dockerLogs.stderr,
+            containsString(
+                "ERROR: File "
+                    + passwordFilename
+                    + " (target of symlink /run/secrets/"
+                    + symlinkFilename
+                    + " from ELASTIC_PASSWORD_FILE) must have file permissions 400 or 600, but actually has: 775"
+            )
+        );
+    }
+
     /**
      * Check that environment variables are translated to -E options even for commands invoked under
      * `docker exec`, where the Docker image's entrypoint is not executed.
      */
-    public void test83EnvironmentVariablesAreRespectedUnderDockerExec() {
+    public void test086EnvironmentVariablesAreRespectedUnderDockerExec() {
         // This test relies on a CLI tool attempting to connect to Elasticsearch, and the
         // tool in question is only in the default distribution.
         assumeTrue(distribution.isDefault());
 
-        runContainer(distribution(), null, Collections.singletonMap("http.host", "this.is.not.valid"));
+        runContainer(distribution(), null, singletonMap("http.host", "this.is.not.valid"));
 
         // This will fail if the env var above is passed as a -E argument
         final Result result = sh.runIgnoreExitCode("elasticsearch-setup-passwords auto");
 
         assertFalse("elasticsearch-setup-passwords command should have failed", result.isSuccess());
-        assertThat(
-            result.stdout,
-            containsString("java.net.UnknownHostException: this.is.not.valid: Name or service not known")
-        );
+        assertThat(result.stdout, containsString("java.net.UnknownHostException: this.is.not.valid: Name or service not known"));
     }
 
     /**