Allow Agent auto auth to read symlinked JWT files

command/agent/auth/jwt/jwt.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
 	"io/ioutil"
 	"net/http"
 	"os"
@@ -31,6 +32,8 @@ type jwtMethod struct {
 	latestToken     *atomic.Value
 }
 
+// NewJWTAuthMethod returns an implementation of Agent's auth.AuthMethod
+// interface for JWT auth.
 func NewJWTAuthMethod(conf *auth.AuthConfig) (auth.AuthMethod, error) {
 	if conf == nil {
 		return nil, errors.New("empty config")
@@ -86,7 +89,7 @@ func NewJWTAuthMethod(conf *auth.AuthConfig) (auth.AuthMethod, error) {
 	return j, nil
 }
 
-func (j *jwtMethod) Authenticate(_ context.Context, client *api.Client) (string, http.Header, map[string]interface{}, error) {
+func (j *jwtMethod) Authenticate(_ context.Context, _ *api.Client) (string, http.Header, map[string]interface{}, error) {
 	j.logger.Trace("beginning authentication")
 
 	j.ingressToken()
@@ -160,8 +163,16 @@ func (j *jwtMethod) ingressToken() {
 
 	j.logger.Debug("new jwt file found")
 
-	if !fi.Mode().IsRegular() {
-		j.logger.Error("jwt file is not a regular file")
+	// Check that the path refers to a file.
+	// If it's a symlink, it could still be a symlink to a directory,
+	// but ioutil.ReadFile below will return a descriptive error.
+	switch mode := fi.Mode(); {
+	case mode.IsRegular():
+		// regular file
+	case mode&fs.ModeSymlink != 0:
+		// symlink
+	default:
+		j.logger.Error("jwt file is not a regular file or symlink")
 		return
 	}
 

command/agent/auth/jwt/jwt_test.go

@@ -0,0 +1,108 @@
+package jwt
+
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"path"
+	"strings"
+	"sync/atomic"
+	"testing"
+
+	"github.com/hashicorp/go-hclog"
+)
+
+func TestIngressToken(t *testing.T) {
+	const (
+		dir       = "dir"
+		file      = "file"
+		empty     = "empty"
+		missing   = "missing"
+		symlinked = "symlinked"
+	)
+
+	rootDir, err := ioutil.TempDir("", "vault-agent-jwt-auth-test")
+	if err != nil {
+		t.Fatalf("failed to create temp dir: %s", err)
+	}
+	defer os.RemoveAll(rootDir)
+
+	setupTestDir := func() string {
+		testDir, err := ioutil.TempDir(rootDir, "")
+		if err != nil {
+			t.Fatal(err)
+		}
+		err = ioutil.WriteFile(path.Join(testDir, file), []byte("test"), 0644)
+		if err != nil {
+			t.Fatal(err)
+		}
+		_, err = os.Create(path.Join(testDir, empty))
+		if err != nil {
+			t.Fatal(err)
+		}
+		err = os.Mkdir(path.Join(testDir, dir), 0755)
+		if err != nil {
+			t.Fatal(err)
+		}
+		err = os.Symlink(path.Join(testDir, file), path.Join(testDir, symlinked))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		return testDir
+	}
+
+	for _, tc := range []struct {
+		name      string
+		path      string
+		errString string
+	}{
+		{
+			"happy path",
+			file,
+			"",
+		},
+		{
+			"path is directory",
+			dir,
+			"[ERROR] jwt file is not a regular file or symlink",
+		},
+		{
+			"path is symlink",
+			symlinked,
+			"",
+		},
+		{
+			"path is missing (implies nothing for ingressToken to do)",
+			missing,
+			"",
+		},
+		{
+			"path is empty file",
+			empty,
+			"[WARN]  empty jwt file read",
+		},
+	} {
+		testDir := setupTestDir()
+		logBuffer := bytes.Buffer{}
+		jwtAuth := &jwtMethod{
+			logger: hclog.New(&hclog.LoggerOptions{
+				Output: &logBuffer,
+			}),
+			latestToken: new(atomic.Value),
+			path:        path.Join(testDir, tc.path),
+		}
+
+		jwtAuth.ingressToken()
+
+		if tc.errString != "" {
+			if !strings.Contains(logBuffer.String(), tc.errString) {
+				t.Fatal("logs did no contain expected error", tc.errString, logBuffer.String())
+			}
+		} else {
+			if strings.Contains(logBuffer.String(), "[ERROR]") || strings.Contains(logBuffer.String(), "[WARN]") {
+				t.Fatal("logs contained unexpected error", logBuffer.String())
+			}
+		}
+	}
+}