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/*
* fs_keyring.go - Add/remove encryption policy keys to/from filesystem
*
* Copyright 2019 Google LLC
* Author: Eric Biggers (ebiggers@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 keyring
/*
#include <string.h>
*/
import "C"
import (
"encoding/hex"
"log"
"os"
"os/user"
"sync"
"unsafe"
"github.com/pkg/errors"
"golang.org/x/sys/unix"
"github.com/google/fscrypt/crypto"
"github.com/google/fscrypt/filesystem"
)
var (
fsKeyringSupported bool
fsKeyringSupportedKnown bool
fsKeyringSupportedLock sync.Mutex
)
func checkForFsKeyringSupport(mount *filesystem.Mount) bool {
dir, err := os.Open(mount.Path)
if err != nil {
log.Printf("Unexpected error opening %q. Assuming filesystem keyring is unsupported.",
mount.Path)
return false
}
defer dir.Close()
// FS_IOC_ADD_ENCRYPTION_KEY with a NULL argument will fail with ENOTTY
// if the ioctl isn't supported. Otherwise it should fail with EFAULT.
//
// Note that there's no need to check for FS_IOC_REMOVE_ENCRYPTION_KEY
// support separately, since it's guaranteed to be available if
// FS_IOC_ADD_ENCRYPTION_KEY is. There's also no need to check for
// support on every filesystem separately, since either the kernel
// supports the ioctls on all fscrypt-capable filesystems or it doesn't.
_, _, errno := unix.Syscall(unix.SYS_IOCTL, dir.Fd(), unix.FS_IOC_ADD_ENCRYPTION_KEY, 0)
if errno == unix.ENOTTY {
log.Printf("Kernel doesn't support filesystem keyring. Falling back to user keyring.")
return false
}
if errno == unix.EFAULT {
log.Printf("Detected support for filesystem keyring")
} else {
// EFAULT is expected, but as long as we didn't get ENOTTY the
// ioctl should be available.
log.Printf("Unexpected error from FS_IOC_ADD_ENCRYPTION_KEY(%q, NULL): %v", mount.Path, errno)
}
return true
}
// isFsKeyringSupported returns true if the kernel supports the ioctls to
// add/remove fscrypt keys directly to/from the filesystem. For support to be
// detected, the given Mount must be for a filesystem that supports fscrypt.
func isFsKeyringSupported(mount *filesystem.Mount) bool {
fsKeyringSupportedLock.Lock()
defer fsKeyringSupportedLock.Unlock()
if !fsKeyringSupportedKnown {
fsKeyringSupported = checkForFsKeyringSupport(mount)
fsKeyringSupportedKnown = true
}
return fsKeyringSupported
}
// buildKeySpecifier converts the key descriptor string to an FscryptKeySpecifier.
func buildKeySpecifier(spec *unix.FscryptKeySpecifier, descriptor string) error {
descriptorBytes, err := hex.DecodeString(descriptor)
if err != nil {
return errors.Errorf("key descriptor %q is invalid", descriptor)
}
switch len(descriptorBytes) {
case unix.FSCRYPT_KEY_DESCRIPTOR_SIZE:
spec.Type = unix.FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR
default:
return errors.Errorf("key descriptor %q has unknown length", descriptor)
}
copy(spec.U[:], descriptorBytes)
return nil
}
// fsAddEncryptionKey adds the specified encryption key to the specified filesystem.
func fsAddEncryptionKey(key *crypto.Key, descriptor string,
mount *filesystem.Mount, user *user.User) error {
dir, err := os.Open(mount.Path)
if err != nil {
return err
}
defer dir.Close()
argKey, err := crypto.NewBlankKey(int(unsafe.Sizeof(unix.FscryptAddKeyArg{})) + key.Len())
if err != nil {
return err
}
defer argKey.Wipe()
arg := (*unix.FscryptAddKeyArg)(argKey.UnsafePtr())
if err = buildKeySpecifier(&arg.Key_spec, descriptor); err != nil {
return err
}
raw := unsafe.Pointer(uintptr(argKey.UnsafePtr()) + unsafe.Sizeof(*arg))
arg.Raw_size = uint32(key.Len())
C.memcpy(raw, key.UnsafePtr(), C.size_t(key.Len()))
_, _, errno := unix.Syscall(unix.SYS_IOCTL, dir.Fd(),
unix.FS_IOC_ADD_ENCRYPTION_KEY, uintptr(argKey.UnsafePtr()))
log.Printf("FS_IOC_ADD_ENCRYPTION_KEY(%q, %s, <raw>) = %v", mount.Path, descriptor, errno)
if errno != 0 {
return errors.Wrap(ErrKeyAdd, errno.Error())
}
return nil
}
// fsRemoveEncryptionKey removes the specified encryption key from the specified
// filesystem.
func fsRemoveEncryptionKey(descriptor string, mount *filesystem.Mount,
user *user.User) error {
dir, err := os.Open(mount.Path)
if err != nil {
return err
}
defer dir.Close()
var arg unix.FscryptRemoveKeyArg
if err = buildKeySpecifier(&arg.Key_spec, descriptor); err != nil {
return err
}
_, _, errno := unix.Syscall(unix.SYS_IOCTL, dir.Fd(),
unix.FS_IOC_REMOVE_ENCRYPTION_KEY, uintptr(unsafe.Pointer(&arg)))
log.Printf("FS_IOC_REMOVE_ENCRYPTION_KEY(%q, %s) = %v, removal_status_flags=0x%x",
mount.Path, descriptor, errno, arg.Removal_status_flags)
switch errno {
case 0:
if arg.Removal_status_flags&unix.FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY != 0 {
return ErrKeyFilesOpen
}
return nil
case unix.ENOKEY:
return ErrKeyNotPresent
default:
return errors.Wrap(ErrKeyRemove, errno.Error())
}
}
// fsGetEncryptionKeyStatus gets the status of the specified encryption key on
// the specified filesystem.
func fsGetEncryptionKeyStatus(descriptor string, mount *filesystem.Mount,
user *user.User) (KeyStatus, error) {
dir, err := os.Open(mount.Path)
if err != nil {
return KeyStatusUnknown, err
}
defer dir.Close()
var arg unix.FscryptGetKeyStatusArg
err = buildKeySpecifier(&arg.Key_spec, descriptor)
if err != nil {
return KeyStatusUnknown, err
}
_, _, errno := unix.Syscall(unix.SYS_IOCTL, dir.Fd(),
unix.FS_IOC_GET_ENCRYPTION_KEY_STATUS, uintptr(unsafe.Pointer(&arg)))
log.Printf("FS_IOC_GET_ENCRYPTION_KEY_STATUS(%q, %s) = %v, status=%d, status_flags=0x%x",
mount.Path, descriptor, errno, arg.Status, arg.Status_flags)
if errno != 0 {
return KeyStatusUnknown, errors.Wrap(ErrKeySearch, errno.Error())
}
switch arg.Status {
case unix.FSCRYPT_KEY_STATUS_ABSENT:
return KeyAbsent, nil
case unix.FSCRYPT_KEY_STATUS_PRESENT:
return KeyPresent, nil
case unix.FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED:
return KeyAbsentButFilesBusy, nil
default:
return KeyStatusUnknown,
errors.Wrapf(ErrKeySearch, "unknown key status (%d)", arg.Status)
}
}
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