func enforceGID(gid, path string) error {
newGID, err := passwd.ParseGID(gid)
if err != nil {
return err
}
// So this is a surprisingly complicated dance if we want to be free of
// potentially hazardous race conditions. We have a path. We can't assume
// anything about its ownership, or mode, whether it's a symlink, etc.
//
// The big risk is that someone is able to create a symlink pointing to
// something they want to illicitly access. Note that since /var/run will
// commonly be used and because this directory is world-writeable, ala /tmp,
// this is a real risk.
//
// So we have to make sure we don't follow symlinks. Assume we are running
// as root (necessary, since we're chowning), and that nothing running as
// root is malicious.
//
// We open the directory as a file so we can modify it using that reference
// without worrying about the resolution of the path changing under us. But
// we need to make sure we don't follow symlinks. This requires special OS
// support, alas.
dir, err := deos.OpenNoSymlinks(path)
if err != nil {
return err
}
defer dir.Close()
fi, err := dir.Stat()
if err != nil {
return err
}
// Attributes of the directory can still change, but its type certainly
// can't. This guarantee is enough for our purposes.
if (fi.Mode() & os.ModeType) != os.ModeDir {
return fmt.Errorf("challenge path %#v is not a directory", path)
}
curUID, err := deos.GetFileUID(fi)
if err != nil {
return err
}
dir.Chmod((fi.Mode() | 0070) & ^os.ModeType) // Ignore errors.
dir.Chown(curUID, newGID) // Ignore errors.
return nil
}
func (h *ihandler) DropPrivileges() error {
if h.dropped {
return nil
}
// Extras
if !h.info.NoBanSuid {
// Try and bansuid, but don't process errors. It may not be supported on
// the current platform, and Linux won't allow SECUREBITS to be set unless
// one is root (or has the right capability). This is basically a
// best-effort thing.
bansuid.BanSuid()
}
// Various fixups
if uidFlag.Value() != "" && gidFlag.Value() == "" {
gid, err := passwd.GetGIDForUID(uidFlag.Value())
if err != nil {
return err
}
gidFlag.SetValue(strconv.FormatInt(int64(gid), 10))
}
if h.info.DefaultChroot == "" {
h.info.DefaultChroot = "/"
}
chrootPath := chrootFlag.Value()
if chrootPath == "" {
chrootPath = h.info.DefaultChroot
}
uid := -1
gid := -1
if uidFlag.Value() != "" {
var err error
uid, err = passwd.ParseUID(uidFlag.Value())
if err != nil {
return err
}
gid, err = passwd.ParseGID(gidFlag.Value())
if err != nil {
return err
}
}
if (uid <= 0) != (gid <= 0) {
return fmt.Errorf("Either both or neither of the UID and GID must be positive")
}
if uid > 0 {
chrootErr, err := daemon.DropPrivileges(uid, gid, chrootPath)
if err != nil {
return fmt.Errorf("Failed to drop privileges: %v", err)
}
if chrootErr != nil && chrootFlag.Value() != "" && chrootFlag.Value() != "/" {
return fmt.Errorf("Failed to chroot: %v", chrootErr)
}
} else if chrootFlag.Value() != "" && chrootFlag.Value() != "/" {
return fmt.Errorf("Must use privilege dropping to use chroot; set -uid")
}
// If we still have any caps (maybe because we didn't setuid), try and drop them.
err := caps.Drop()
if err != nil {
return fmt.Errorf("cannot drop caps: %v", err)
}
if !h.info.AllowRoot && daemon.IsRoot() {
return fmt.Errorf("Daemon must not run as root or with capabilities; run as non-root user or use -uid")
}
h.dropped = true
return nil
}