chmod race in doUidshiftIntoContainer()

doUidshiftIntoContainer() has an unsafe Chmod() call that races against
the stat in the Filepath.Walk() function. A symbolic link created in
that window could cause any file on the system to have any mode of the
attacker's choice.

func (set *IdmapSet) doUidshiftIntoContainer(dir string, testmode bool, how string) error {
        convert := func(path string, fi os.FileInfo, err error) (e error) {
                uid, gid, err := GetOwner(path)
                if err != nil {
                        return err
                }
                var newuid, newgid int
                switch how {
                case "in":
                        newuid, newgid = set.ShiftIntoNs(uid, gid)
                case "out":
                        newuid, newgid = set.ShiftFromNs(uid, gid)
                }
                if testmode {
                        fmt.Printf("I would shift %q to %d %d\n", path, newuid, newgid)
                } else {
                        err = os.Lchown(path, int(newuid), int(newgid))
                        if err == nil {
                                m := fi.Mode()
                                if m&os.ModeSymlink == 0 {
                                        err = os.Chmod(path, m)
                                        if err != nil {
                                                fmt.Printf("Error resetting mode on %q, continuing\n", path)
                                        }
                                }
                        }
                }
                return nil
        }

        if !PathExists(dir) {
                return fmt.Errorf("No such file or directory: %q", dir)
        }
        return filepath.Walk(dir, convert)
}

This function cannot be used if any processes from the uid ranges in
question could be executing on the system, or on any systems that may mount
the target filesystem via nfs, p9fs, or other networked filesystems.

The mode change needs to be done after dropping privileges to the target
user and group so that the chmod can only affect files owned by the uid
in question.

Thanks