hls: prevent some CodeQL warnings (#6024)

internal/servers/hls/muxer_instance.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -25,6 +26,26 @@ const (
 	sessionCleanupPeriod  = sessionCloseAfter / 3
 )
 
+// this prevents directory traversal.
+// functionally it's useless since there's already conf.IsPathName, but it's needed by CodeQL.
+func safeSubDirectory(base string, pathName string) (string, error) {
+	baseAbs, err := filepath.Abs(filepath.Clean(base))
+	if err != nil {
+		return "", err
+	}
+
+	candidateAbs, err := filepath.Abs(filepath.Join(baseAbs, pathName))
+	if err != nil {
+		return "", err
+	}
+
+	if !strings.HasPrefix(candidateAbs, baseAbs) {
+		return "", fmt.Errorf("path escapes base directory")
+	}
+
+	return candidateAbs, nil
+}
+
 type instanceParent interface {
 	logger.Writer
 	closeInstance(*muxerInstance, error)
@@ -54,9 +75,18 @@ func (mi *muxerInstance) initialize() error {
 	mi.Log(logger.Debug, "instance created")
 
 	var muxerDirectory string
+
 	if mi.directory != "" {
-		muxerDirectory = filepath.Join(mi.directory, mi.pathName)
-		os.MkdirAll(muxerDirectory, 0o755)
+		var err error
+		muxerDirectory, err = safeSubDirectory(mi.directory, mi.pathName)
+		if err != nil {
+			return err
+		}
+
+		err = os.MkdirAll(muxerDirectory, 0o755)
+		if err != nil {
+			return err
+		}
 	}
 
 	mi.hmuxer = &gohlslib.Muxer{

internal/servers/hls/server_test.go

@@ -538,6 +538,20 @@ func TestServerDirectory(t *testing.T) {
 	require.NoError(t, err)
 }
 
+func TestSafeSubDirectory(t *testing.T) {
+	base := t.TempDir()
+
+	path, err := safeSubDirectory(base, "group/cam1")
+	require.NoError(t, err)
+	require.Equal(t, filepath.Join(base, "group", "cam1"), path)
+
+	_, err = safeSubDirectory(base, "../cam1")
+	require.Error(t, err)
+
+	_, err = safeSubDirectory(base, "group/../../cam1")
+	require.Error(t, err)
+}
+
 func TestServerDynamicAlwaysRemux(t *testing.T) {
 	desc := &description.Session{Medias: []*description.Media{test.MediaH264}}