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/*
* keyring.go - Handles inserting/removing into user keyrings.
*
* Copyright 2017 Google Inc.
* Author: Joe Richey (joerichey@google.com)
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy of
* the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations under
* the License.
*/
package security
import (
"fmt"
"log"
"os/user"
"sync"
"github.com/pkg/errors"
"golang.org/x/sys/unix"
"github.com/google/fscrypt/util"
)
// KeyType is always logon as required by filesystem encryption.
const KeyType = "logon"
// Keyring related error values
var (
ErrKeySearch = errors.New("could not find key with descriptor")
ErrKeyRemove = util.SystemError("could not remove key from the keyring")
ErrKeyInsert = util.SystemError("could not insert key into the keyring")
ErrSessionUserKeying = errors.New("user keyring not linked into session keyring")
ErrAccessUserKeyring = errors.New("could not access user keyring")
ErrLinkUserKeyring = util.SystemError("could not link user keyring into root keyring")
)
// FindKey tries to locate a key in the kernel keyring with the provided
// description. The key ID is returned if we can find the key. An error is
// returned if the key does not exist.
func FindKey(description string, target *user.User) (int, error) {
keyringID, err := UserKeyringID(target, false)
if err != nil {
return 0, err
}
keyID, err := unix.KeyctlSearch(keyringID, KeyType, description, 0)
log.Printf("KeyctlSearch(%d, %s, %s) = %d, %v", keyringID, KeyType, description, keyID, err)
if err != nil {
return 0, errors.Wrap(ErrKeySearch, err.Error())
}
return keyID, err
}
// RemoveKey tries to remove a policy key from the kernel keyring with the
// provided description. An error is returned if the key does not exist.
func RemoveKey(description string, target *user.User) error {
keyID, err := FindKey(description, target)
if err != nil {
return err
}
// We use KEYCTL_INVALIDATE instead of KEYCTL_REVOKE because
// invalidating a key immediately removes it.
_, err = unix.KeyctlInt(unix.KEYCTL_INVALIDATE, keyID, 0, 0, 0)
log.Printf("KeyctlInvalidate(%d) = %v", keyID, err)
if err != nil {
return errors.Wrap(ErrKeyRemove, err.Error())
}
return nil
}
// InsertKey puts the provided data into the kernel keyring with the provided
// description.
func InsertKey(data []byte, description string, target *user.User) error {
keyringID, err := UserKeyringID(target, true)
if err != nil {
return err
}
keyID, err := unix.AddKey(KeyType, description, data, keyringID)
log.Printf("KeyctlAddKey(%s, %s, <data>, %d) = %d, %v",
KeyType, description, keyringID, keyID, err)
if err != nil {
return errors.Wrap(ErrKeyInsert, err.Error())
}
return nil
}
var (
keyringIDCache = make(map[int]int)
cacheLock sync.Mutex
)
// UserKeyringID returns the key id of the target user's user keyring. We also
// ensure that the keyring will be accessible by linking it into the process
// keyring and linking it into the root user keyring (permissions allowing). If
// checkSession is true, an error is returned if a normal user requests their
// user keyring, but it is not in the current session keyring.
func UserKeyringID(target *user.User, checkSession bool) (int, error) {
uid := util.AtoiOrPanic(target.Uid)
targetKeyring, err := userKeyringIDLookup(uid)
if err != nil {
return 0, errors.Wrap(ErrAccessUserKeyring, err.Error())
}
if !util.IsUserRoot() {
// Make sure the returned keyring will be accessible by checking
// that it is in the session keyring.
if checkSession && !isUserKeyringInSession(uid) {
return 0, ErrSessionUserKeying
}
return targetKeyring, nil
}
// Make sure the returned keyring will be accessible by linking it into
// the root user's user keyring (which will not be garbage collected).
rootKeyring, err := userKeyringIDLookup(0)
if err != nil {
return 0, errors.Wrap(ErrLinkUserKeyring, err.Error())
}
if rootKeyring != targetKeyring {
if err = keyringLink(targetKeyring, rootKeyring); err != nil {
return 0, errors.Wrap(ErrLinkUserKeyring, err.Error())
}
}
return targetKeyring, nil
}
func userKeyringIDLookup(uid int) (keyringID int, err error) {
cacheLock.Lock()
defer cacheLock.Unlock()
var ok bool
if keyringID, ok = keyringIDCache[uid]; ok {
return
}
// Our goals here are to:
// - Find the user keyring (for the provided uid)
// - Link it into the current process keyring (so we can use it)
// - Make no permanent changes to the process privileges
// Complicating this are the facts that:
// - The value of KEY_SPEC_USER_KEYRING is determined by the ruid
// - Keyring linking permissions use the euid
// So we have to change both the ruid and euid to make this work,
// setting the suid to 0 so that we can later switch back.
ruid, euid, suid := getUids()
if ruid != uid || euid != uid {
if err = setUids(uid, uid, 0); err != nil {
return
}
defer func() {
resetErr := setUids(ruid, euid, suid)
if resetErr != nil {
err = resetErr
}
}()
}
// We get the value of KEY_SPEC_USER_KEYRING. Note that this will also
// trigger the creation of the uid keyring if it does not yet exist.
keyringID, err = unix.KeyctlGetKeyringID(unix.KEY_SPEC_USER_KEYRING, true)
log.Printf("keyringID(_uid.%d) = %d, %v", uid, keyringID, err)
if err != nil {
return 0, err
}
// We still want to use this keyring after our privileges are reset. So
// we link it into the process keyring, preventing a loss of access.
if err = keyringLink(keyringID, unix.KEY_SPEC_PROCESS_KEYRING); err != nil {
return 0, err
}
keyringIDCache[uid] = keyringID
return keyringID, nil
}
// isUserKeyringInSession tells us if the user's uid keyring is in the current
// session keyring.
func isUserKeyringInSession(uid int) bool {
// We cannot use unix.KEY_SPEC_SESSION_KEYRING directly as that might
// create a session keyring if one does not exist.
sessionKeyring, err := unix.KeyctlGetKeyringID(unix.KEY_SPEC_SESSION_KEYRING, false)
log.Printf("keyringID(session) = %d, %v", sessionKeyring, err)
if err != nil {
return false
}
description := fmt.Sprintf("_uid.%d", uid)
id, err := unix.KeyctlSearch(sessionKeyring, "keyring", description, 0)
log.Printf("KeyctlSearch(%d, keyring, %s) = %d, %v", sessionKeyring, description, id, err)
return err == nil
}
func keyringLink(keyID int, keyringID int) error {
_, err := unix.KeyctlInt(unix.KEYCTL_LINK, keyID, keyringID, 0, 0)
log.Printf("KeyctlLink(%d, %d) = %v", keyID, keyringID, err)
return err
}
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