overlayFS: Fix lock ordering/race in SetStat.

pkg/sentry/fsimpl/overlay/overlay.go

@@ -922,6 +922,7 @@ func (d *dentry) statInternalTo(ctx context.Context, opts *vfs.StatOptions, stat
 }
 
 // Preconditions: d.copyMu must be locked for writing.
+// +checklocks:d.copyMu
 func (d *dentry) updateAfterSetStatLocked(opts *vfs.SetStatOptions) {
 	if opts.Stat.Mask&linux.STATX_MODE != 0 {
 		// If the mode was changed, the underlying file's ACL may have changed,

pkg/sentry/fsimpl/overlay/regular_file.go

@@ -70,6 +70,7 @@ func (fd *regularFileFD) getCurrentFD(ctx context.Context) (*vfs.FileDescription
 	return wrappedFD, nil
 }
 
+// +checklocks:fd.mu
 func (fd *regularFileFD) currentFDLocked(ctx context.Context) (*vfs.FileDescription, error) {
 	d := fd.dentry()
 	statusFlags := fd.vfsfd.StatusFlags()
@@ -175,17 +176,24 @@ func (fd *regularFileFD) SetStat(ctx context.Context, opts vfs.SetStatOptions) e
 		return err
 	}
 	defer mnt.EndWrite()
-	if err := d.copyUpLocked(ctx); err != nil {
-		return err
+	// d.copiedUp is never cleared once set, so this check can be safely
+	// performed without holding locks.
+	if !d.isCopiedUp() {
+		d.fs.renameMu.RLock()
+		err := d.copyUpLocked(ctx)
+		d.fs.renameMu.RUnlock()
+		if err != nil {
+			return err
+		}
 	}
-	// Changes to d's attributes are serialized by d.copyMu.
-	d.copyMu.Lock()
-	defer d.copyMu.Unlock()
 	wrappedFD, err := fd.getCurrentFD(ctx)
 	if err != nil {
 		return err
 	}
 	defer wrappedFD.DecRef(ctx)
+	// Changes to d's attributes are serialized by d.copyMu.
+	d.copyMu.Lock()
+	defer d.copyMu.Unlock()
 	if err := wrappedFD.SetStat(ctx, opts); err != nil {
 		return err
 	}