// UnpackWithDropPrivs will unapck the ClickDeb content into the
// target dir and drop privs when doing this.
//
// To do this reliably in go we need to exec a helper as we can not
// just fork() and drop privs in the child (no support for stock fork in go)
func (d *ClickDeb) UnpackWithDropPrivs(instDir, rootdir string) error {
// no need to drop privs, we are not root
if !helpers.ShouldDropPrivs() {
return d.Unpack(instDir)
}
cmd := exec.Command("snappy", "internal-unpack", d.Name(), instDir, rootdir)
cmd.Stdout = os.Stdout
cmd.Stderr = os.Stderr
if err := cmd.Run(); err != nil {
return &ErrUnpackFailed{
snapFile: d.Name(),
instDir: instDir,
origErr: err,
}
}
return nil
}
func unpackAndDropPrivs(snapFile, targetDir, rootDir string) error {
d, err := clickdeb.Open(snapFile)
if err != nil {
return err
}
defer d.Close()
if helpers.ShouldDropPrivs() {
var dropPrivsUser string
// first find out what user to use
passFile := passwdFile(rootDir, "passwd")
for _, dropPrivsUser = range dropPrivsUsers {
_, err := readUID(dropPrivsUser, passFile)
if err == nil {
break
}
}
// then get uid/gid
uid, err := readUID(dropPrivsUser, passFile)
if err != nil {
return err
}
groupFile := passwdFile(rootDir, "group")
gid, err := readUID(dropPrivsUser, groupFile)
if err != nil {
return err
}
if err := os.MkdirAll(targetDir, 0755); err != nil {
return err
}
if err := os.Chown(targetDir, uid, gid); err != nil {
return err
}
// Setuid and Setgid only apply to the current Linux thread, so make
// sure we don't get moved.
runtime.LockOSThread()
// run prctl(PR_SET_NO_NEW_PRIVS)
rc := C.prctl_no_new_privs()
if rc < 0 {
return fmt.Errorf("prctl(PR_SET_NO_NEW_PRIVS) failed with %v", rc)
}
if err := syscall.Setgroups([]int{gid}); err != nil {
return fmt.Errorf("Setgroups([]{%d}) call failed: %v", gid, err)
}
if err := setgid(gid); err != nil {
return fmt.Errorf("Setgid(%d) call failed: %v", gid, err)
}
if err := setuid(uid); err != nil {
return fmt.Errorf("Setuid(%d) call failed: %v", uid, err)
}
// extra paranoia
if syscall.Getuid() != uid || syscall.Getgid() != gid {
return fmt.Errorf("Dropping privileges failed, uid is %v, gid is %v", syscall.Getuid(), syscall.Getgid())
}
}
return d.UnpackAll(targetDir)
}