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/*
* privileges.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 (
"log"
"os"
"os/user"
"runtime"
"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 (
ErrFindingKeyring = util.SystemError("could not find user keyring")
ErrKeyringInsert = util.SystemError("could not insert key into the keyring")
ErrKeyringSearch = errors.New("could not find key with descriptor")
ErrKeyringDelete = util.SystemError("could not delete key from the keyring")
ErrKeyringLink = util.SystemError("could not link keyring")
)
// KeyringsSetup configures the desired keyring linkage by linking the target
// user's keying into the privileged user's keyring.
func KeyringsSetup(target, privileged *user.User) error {
targetKeyringID, err := userKeyringID(target)
if err != nil {
return err
}
privilegedKeyringID, err := userKeyringID(privileged)
if err != nil {
return err
}
return keyringLink(targetKeyringID, privilegedKeyringID)
}
// 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)
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(ErrKeyringSearch, 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(ErrKeyringDelete, 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)
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(ErrKeyringInsert, err.Error())
}
return nil
}
var (
keyringIDCache = make(map[int]int)
cacheLock sync.Mutex
)
// userKeyringID returns the key id of the target user's keyring. The returned
// keyring will also be linked into the process keyring so that it will be
// accessible thoughout the program.
func userKeyringID(target *user.User) (int, error) {
uid := util.AtoiOrPanic(target.Uid)
// We will cache the result of this function.
cacheLock.Lock()
defer cacheLock.Unlock()
if keyringID, ok := keyringIDCache[uid]; ok {
return keyringID, nil
}
// The permissions of the keyrings API is a little strange. The euid is
// used to determine if we can access/modify a key/keyring. However, the
// ruid is used to determine KEY_SPEC_USER_KEYRING. This means both the
// ruid and euid must match the user's uid for the lookup to work.
if uid == os.Getuid() && uid == os.Geteuid() {
log.Printf("Normal keyring lookup for uid=%d", uid)
return userKeyringIDLookup(uid)
}
// We drop permissions in a separate thread (guaranteed as the main
// thread is locked) because we need to drop the real AND effective IDs.
log.Printf("Threaded keyring lookup for uid=%d", uid)
idChan := make(chan int, 0)
errChan := make(chan error, 0)
// OSThread locks ensure the privilege change is only for the lookup.
runtime.LockOSThread()
defer runtime.UnlockOSThread()
go func() {
runtime.LockOSThread()
if err := SetThreadPrivileges(target, true); err != nil {
errChan <- err
return
}
keyringID, err := userKeyringIDLookup(uid)
if err != nil {
errChan <- err
return
}
idChan <- keyringID
return
}()
// We select so the thread will have to complete
select {
case err := <-errChan:
return 0, err
case keyringID := <-idChan:
if uid == os.Getuid() && uid == os.Geteuid() {
log.Print("thread privileges now incorrect")
}
return keyringID, nil
}
}
func userKeyringIDLookup(uid int) (int, error) {
// This will trigger the creation of the user keyring, if necessary.
keyringID, err := unix.KeyctlGetKeyringID(unix.KEY_SPEC_USER_KEYRING, false)
log.Printf("keyringID(_uid.%d) = %d, %v", uid, keyringID, err)
if err != nil {
return 0, errors.Wrap(ErrFindingKeyring, err.Error())
}
// For some silly reason, a thread does not automatically "possess" keys
// in the user keyring. So we link it into the process keyring so that
// we will not get "permission denied" when purging or modifying keys.
if err := keyringLink(keyringID, unix.KEY_SPEC_PROCESS_KEYRING); err != nil {
return 0, err
}
keyringIDCache[uid] = keyringID
return keyringID, 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)
if err != nil {
return errors.Wrap(ErrKeyringLink, err.Error())
}
return nil
}
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