xref: /Documentation/filesystems/fscrypt.rst

=====================================
Filesystem-level encryption (fscrypt)
=====================================

Introduction
============

fscrypt is a library which filesystems can hook into to support
transparent encryption of files and directories.

Note: "fscrypt" in this document refers to the kernel-level portion,
implemented in ``fs/crypto/``, as opposed to the userspace tool
`fscrypt <https://github.com/google/fscrypt>`_.  This document only
covers the kernel-level portion.  For command-line examples of how to
use encryption, see the documentation for the userspace tool `fscrypt
<https://github.com/google/fscrypt>`_.  Also, it is recommended to use
the fscrypt userspace tool, or other existing userspace tools such as
`fscryptctl <https://github.com/google/fscryptctl>`_ or `Android's key
management system
<https://source.android.com/security/encryption/file-based>`_, over
using the kernel's API directly.  Using existing tools reduces the
chance of introducing your own security bugs.  (Nevertheless, for
completeness this documentation covers the kernel's API anyway.)

Unlike dm-crypt, fscrypt operates at the filesystem level rather than
at the block device level.  This allows it to encrypt different files
with different keys and to have unencrypted files on the same
filesystem.  This is useful for multi-user systems where each user's
data-at-rest needs to be cryptographically isolated from the others.
However, except for filenames, fscrypt does not encrypt filesystem
metadata.

Unlike eCryptfs, which is a stacked filesystem, fscrypt is integrated
directly into supported filesystems --- currently ext4, F2FS, and
UBIFS.  This allows encrypted files to be read and written without
caching both the decrypted and encrypted pages in the pagecache,
thereby nearly halving the memory used and bringing it in line with
unencrypted files.  Similarly, half as many dentries and inodes are
needed.  eCryptfs also limits encrypted filenames to 143 bytes,
causing application compatibility issues; fscrypt allows the full 255
bytes (NAME_MAX).  Finally, unlike eCryptfs, the fscrypt API can be
used by unprivileged users, with no need to mount anything.

fscrypt does not support encrypting files in-place.  Instead, it
supports marking an empty directory as encrypted.  Then, after
userspace provides the key, all regular files, directories, and
symbolic links created in that directory tree are transparently
encrypted.

Threat model
============

Offline attacks
---------------

Provided that userspace chooses a strong encryption key, fscrypt
protects the confidentiality of file contents and filenames in the
event of a single point-in-time permanent offline compromise of the
block device content.  fscrypt does not protect the confidentiality of
non-filename metadata, e.g. file sizes, file permissions, file
timestamps, and extended attributes.  Also, the existence and location
of holes (unallocated blocks which logically contain all zeroes) in
files is not protected.

fscrypt is not guaranteed to protect confidentiality or authenticity
if an attacker is able to manipulate the filesystem offline prior to
an authorized user later accessing the filesystem.

Online attacks
--------------

fscrypt (and storage encryption in general) can only provide limited
protection, if any at all, against online attacks.  In detail:

Side-channel attacks
~~~~~~~~~~~~~~~~~~~~

fscrypt is only resistant to side-channel attacks, such as timing or
electromagnetic attacks, to the extent that the underlying Linux
Cryptographic API algorithms or inline encryption hardware are.  If a
vulnerable algorithm is used, such as a table-based implementation of
AES, it may be possible for an attacker to mount a side channel attack
against the online system.  Side channel attacks may also be mounted
against applications consuming decrypted data.

Unauthorized file access
~~~~~~~~~~~~~~~~~~~~~~~~

After an encryption key has been added, fscrypt does not hide the
plaintext file contents or filenames from other users on the same
system.  Instead, existing access control mechanisms such as file mode
bits, POSIX ACLs, LSMs, or namespaces should be used for this purpose.

(For the reasoning behind this, understand that while the key is
added, the confidentiality of the data, from the perspective of the
system itself, is *not* protected by the mathematical properties of
encryption but rather only by the correctness of the kernel.
Therefore, any encryption-specific access control checks would merely
be enforced by kernel *code* and therefore would be largely redundant
with the wide variety of access control mechanisms already available.)

Kernel memory compromise
~~~~~~~~~~~~~~~~~~~~~~~~

An attacker who compromises the system enough to read from arbitrary
memory, e.g. by mounting a physical attack or by exploiting a kernel
security vulnerability, can compromise all encryption keys that are
currently in use.

However, fscrypt allows encryption keys to be removed from the kernel,
which may protect them from later compromise.

In more detail, the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl (or the
FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl) can wipe a master
encryption key from kernel memory.  If it does so, it will also try to
evict all cached inodes which had been "unlocked" using the key,
thereby wiping their per-file keys and making them once again appear
"locked", i.e. in ciphertext or encrypted form.

However, these ioctls have some limitations:

- Per-file keys for in-use files will *not* be removed or wiped.
  Therefore, for maximum effect, userspace should close the relevant
  encrypted files and directories before removing a master key, as
  well as kill any processes whose working directory is in an affected
  encrypted directory.

- The kernel cannot magically wipe copies of the master key(s) that
  userspace might have as well.  Therefore, userspace must wipe all
  copies of the master key(s) it makes as well; normally this should
  be done immediately after FS_IOC_ADD_ENCRYPTION_KEY, without waiting
  for FS_IOC_REMOVE_ENCRYPTION_KEY.  Naturally, the same also applies
  to all higher levels in the key hierarchy.  Userspace should also
  follow other security precautions such as mlock()ing memory
  containing keys to prevent it from being swapped out.

- In general, decrypted contents and filenames in the kernel VFS
  caches are freed but not wiped.  Therefore, portions thereof may be
  recoverable from freed memory, even after the corresponding key(s)
  were wiped.  To partially solve this, you can set
  CONFIG_PAGE_POISONING=y in your kernel config and add page_poison=1
  to your kernel command line.  However, this has a performance cost.

- Secret keys might still exist in CPU registers, in crypto
  accelerator hardware (if used by the crypto API to implement any of
  the algorithms), or in other places not explicitly considered here.

Limitations of v1 policies
~~~~~~~~~~~~~~~~~~~~~~~~~~

v1 encryption policies have some weaknesses with respect to online
attacks:

- There is no verification that the provided master key is correct.
  Therefore, a malicious user can temporarily associate the wrong key
  with another user's encrypted files to which they have read-only
  access.  Because of filesystem caching, the wrong key will then be
  used by the other user's accesses to those files, even if the other
  user has the correct key in their own keyring.  This violates the
  meaning of "read-only access".

- A compromise of a per-file key also compromises the master key from
  which it was derived.

- Non-root users cannot securely remove encryption keys.

All the above problems are fixed with v2 encryption policies.  For
this reason among others, it is recommended to use v2 encryption
policies on all new encrypted directories.

Key hierarchy
=============

Master Keys
-----------

Each encrypted directory tree is protected by a *master key*.  Master
keys can be up to 64 bytes long, and must be at least as long as the
greater of the security strength of the contents and filenames
encryption modes being used.  For example, if any AES-256 mode is
used, the master key must be at least 256 bits, i.e. 32 bytes.  A
stricter requirement applies if the key is used by a v1 encryption
policy and AES-256-XTS is used; such keys must be 64 bytes.

To "unlock" an encrypted directory tree, userspace must provide the
appropriate master key.  There can be any number of master keys, each
of which protects any number of directory trees on any number of
filesystems.

Master keys must be real cryptographic keys, i.e. indistinguishable
from random bytestrings of the same length.  This implies that users
**must not** directly use a password as a master key, zero-pad a
shorter key, or repeat a shorter key.  Security cannot be guaranteed
if userspace makes any such error, as the cryptographic proofs and
analysis would no longer apply.

Instead, users should generate master keys either using a
cryptographically secure random number generator, or by using a KDF
(Key Derivation Function).  The kernel does not do any key stretching;
therefore, if userspace derives the key from a low-entropy secret such
as a passphrase, it is critical that a KDF designed for this purpose
be used, such as scrypt, PBKDF2, or Argon2.

Key derivation function
-----------------------

With one exception, fscrypt never uses the master key(s) for
encryption directly.  Instead, they are only used as input to a KDF
(Key Derivation Function) to derive the actual keys.

The KDF used for a particular master key differs depending on whether
the key is used for v1 encryption policies or for v2 encryption
policies.  Users **must not** use the same key for both v1 and v2
encryption policies.  (No real-world attack is currently known on this
specific case of key reuse, but its security cannot be guaranteed
since the cryptographic proofs and analysis would no longer apply.)

For v1 encryption policies, the KDF only supports deriving per-file
encryption keys.  It works by encrypting the master key with
AES-128-ECB, using the file's 16-byte nonce as the AES key.  The
resulting ciphertext is used as the derived key.  If the ciphertext is
longer than needed, then it is truncated to the needed length.

For v2 encryption policies, the KDF is HKDF-SHA512.  The master key is
passed as the "input keying material", no salt is used, and a distinct
"application-specific information string" is used for each distinct
key to be derived.  For example, when a per-file encryption key is
derived, the application-specific information string is the file's
nonce prefixed with "fscrypt\\0" and a context byte.  Different
context bytes are used for other types of derived keys.

HKDF-SHA512 is preferred to the original AES-128-ECB based KDF because
HKDF is more flexible, is nonreversible, and evenly distributes
entropy from the master key.  HKDF is also standardized and widely
used by other software, whereas the AES-128-ECB based KDF is ad-hoc.

Per-file encryption keys
------------------------

Since each master key can protect many files, it is necessary to
"tweak" the encryption of each file so that the same plaintext in two
files doesn't map to the same ciphertext, or vice versa.  In most
cases, fscrypt does this by deriving per-file keys.  When a new
encrypted inode (regular file, directory, or symlink) is created,
fscrypt randomly generates a 16-byte nonce and stores it in the
inode's encryption xattr.  Then, it uses a KDF (as described in `Key
derivation function`_) to derive the file's key from the master key
and nonce.

Key derivation was chosen over key wrapping because wrapped keys would
require larger xattrs which would be less likely to fit in-line in the
filesystem's inode table, and there didn't appear to be any
significant advantages to key wrapping.  In particular, currently
there is no requirement to support unlocking a file with multiple
alternative master keys or to support rotating master keys.  Instead,
the master keys may be wrapped in userspace, e.g. as is done by the
`fscrypt <https://github.com/google/fscrypt>`_ tool.

DIRECT_KEY policies
-------------------

The Adiantum encryption mode (see `Encryption modes and usage`_) is
suitable for both contents and filenames encryption, and it accepts
long IVs --- long enough to hold both an 8-byte data unit index and a
16-byte per-file nonce.  Also, the overhead of each Adiantum key is
greater than that of an AES-256-XTS key.

Therefore, to improve performance and save memory, for Adiantum a
"direct key" configuration is supported.  When the user has enabled
this by setting FSCRYPT_POLICY_FLAG_DIRECT_KEY in the fscrypt policy,
per-file encryption keys are not used.  Instead, whenever any data
(contents or filenames) is encrypted, the file's 16-byte nonce is
included in the IV.  Moreover:

- For v1 encryption policies, the encryption is done directly with the
  master key.  Because of this, users **must not** use the same master
  key for any other purpose, even for other v1 policies.

- For v2 encryption policies, the encryption is done with a per-mode
  key derived using the KDF.  Users may use the same master key for
  other v2 encryption policies.

IV_INO_LBLK_64 policies
-----------------------

When FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64 is set in the fscrypt policy,
the encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.  This normally results in all files
protected by the same master key sharing a single contents encryption
key and a single filenames encryption key.  To still encrypt different
files' data differently, inode numbers are included in the IVs.
Consequently, shrinking the filesystem may not be allowed.

This format is optimized for use with inline encryption hardware
compliant with the UFS standard, which supports only 64 IV bits per
I/O request and may have only a small number of keyslots.

IV_INO_LBLK_32 policies
-----------------------

IV_INO_LBLK_32 policies work like IV_INO_LBLK_64, except that for
IV_INO_LBLK_32, the inode number is hashed with SipHash-2-4 (where the
SipHash key is derived from the master key) and added to the file data
unit index mod 2^32 to produce a 32-bit IV.

This format is optimized for use with inline encryption hardware
compliant with the eMMC v5.2 standard, which supports only 32 IV bits
per I/O request and may have only a small number of keyslots.  This
format results in some level of IV reuse, so it should only be used
when necessary due to hardware limitations.

Key identifiers
---------------

For master keys used for v2 encryption policies, a unique 16-byte "key
identifier" is also derived using the KDF.  This value is stored in
the clear, since it is needed to reliably identify the key itself.

Dirhash keys
------------

For directories that are indexed using a secret-keyed dirhash over the
plaintext filenames, the KDF is also used to derive a 128-bit
SipHash-2-4 key per directory in order to hash filenames.  This works
just like deriving a per-file encryption key, except that a different
KDF context is used.  Currently, only casefolded ("case-insensitive")
encrypted directories use this style of hashing.

Encryption modes and usage
==========================

fscrypt allows one encryption mode to be specified for file contents
and one encryption mode to be specified for filenames.  Different
directory trees are permitted to use different encryption modes.
Currently, the following pairs of encryption modes are supported:

- AES-256-XTS for contents and AES-256-CTS-CBC for filenames
- AES-128-CBC for contents and AES-128-CTS-CBC for filenames
- Adiantum for both contents and filenames
- AES-256-XTS for contents and AES-256-HCTR2 for filenames (v2 policies only)
- SM4-XTS for contents and SM4-CTS-CBC for filenames (v2 policies only)

If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.

AES-128-CBC was added only for low-powered embedded devices with
crypto accelerators such as CAAM or CESA that do not support XTS.  To
use AES-128-CBC, CONFIG_CRYPTO_ESSIV and CONFIG_CRYPTO_SHA256 (or
another SHA-256 implementation) must be enabled so that ESSIV can be
used.

Adiantum is a (primarily) stream cipher-based mode that is fast even
on CPUs without dedicated crypto instructions.  It's also a true
wide-block mode, unlike XTS.  It can also eliminate the need to derive
per-file encryption keys.  However, it depends on the security of two
primitives, XChaCha12 and AES-256, rather than just one.  See the
paper "Adiantum: length-preserving encryption for entry-level
processors" (https://eprint.iacr.org/2018/720.pdf) for more details.
To use Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled.  Also, fast
implementations of ChaCha and NHPoly1305 should be enabled, e.g.
CONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM.

AES-256-HCTR2 is another true wide-block encryption mode that is intended for
use on CPUs with dedicated crypto instructions.  AES-256-HCTR2 has the property
that a bitflip in the plaintext changes the entire ciphertext.  This property
makes it desirable for filename encryption since initialization vectors are
reused within a directory.  For more details on AES-256-HCTR2, see the paper
"Length-preserving encryption with HCTR2"
(https://eprint.iacr.org/2021/1441.pdf).  To use AES-256-HCTR2,
CONFIG_CRYPTO_HCTR2 must be enabled.  Also, fast implementations of XCTR and
POLYVAL should be enabled, e.g. CRYPTO_POLYVAL_ARM64_CE and
CRYPTO_AES_ARM64_CE_BLK for ARM64.

SM4 is a Chinese block cipher that is an alternative to AES.  It has
not seen as much security review as AES, and it only has a 128-bit key
size.  It may be useful in cases where its use is mandated.
Otherwise, it should not be used.  For SM4 support to be available, it
also needs to be enabled in the kernel crypto API.

New encryption modes can be added relatively easily, without changes
to individual filesystems.  However, authenticated encryption (AE)
modes are not currently supported because of the difficulty of dealing
with ciphertext expansion.

Contents encryption
-------------------

For contents encryption, each file's contents is divided into "data
units".  Each data unit is encrypted independently.  The IV for each
data unit incorporates the zero-based index of the data unit within
the file.  This ensures that each data unit within a file is encrypted
differently, which is essential to prevent leaking information.

Note: the encryption depending on the offset into the file means that
operations like "collapse range" and "insert range" that rearrange the
extent mapping of files are not supported on encrypted files.

There are two cases for the sizes of the data units:

* Fixed-size data units.  This is how all filesystems other than UBIFS
  work.  A file's data units are all the same size; the last data unit
  is zero-padded if needed.  By default, the data unit size is equal
  to the filesystem block size.  On some filesystems, users can select
  a sub-block data unit size via the ``log2_data_unit_size`` field of
  the encryption policy; see `FS_IOC_SET_ENCRYPTION_POLICY`_.

* Variable-size data units.  This is what UBIFS does.  Each "UBIFS
  data node" is treated as a crypto data unit.  Each contains variable
  length, possibly compressed data, zero-padded to the next 16-byte
  boundary.  Users cannot select a sub-block data unit size on UBIFS.

In the case of compression + encryption, the compressed data is
encrypted.  UBIFS compression works as described above.  f2fs
compression works a bit differently; it compresses a number of
filesystem blocks into a smaller number of filesystem blocks.
Therefore a f2fs-compressed file still uses fixed-size data units, and
it is encrypted in a similar way to a file containing holes.

As mentioned in `Key hierarchy`_, the default encryption setting uses
per-file keys.  In this case, the IV for each data unit is simply the
index of the data unit in the file.  However, users can select an
encryption setting that does not use per-file keys.  For these, some
kind of file identifier is incorporated into the IVs as follows:

- With `DIRECT_KEY policies`_, the data unit index is placed in bits
  0-63 of the IV, and the file's nonce is placed in bits 64-191.

- With `IV_INO_LBLK_64 policies`_, the data unit index is placed in
  bits 0-31 of the IV, and the file's inode number is placed in bits
  32-63.  This setting is only allowed when data unit indices and
  inode numbers fit in 32 bits.

- With `IV_INO_LBLK_32 policies`_, the file's inode number is hashed
  and added to the data unit index.  The resulting value is truncated
  to 32 bits and placed in bits 0-31 of the IV.  This setting is only
  allowed when data unit indices and inode numbers fit in 32 bits.

The byte order of the IV is always little endian.

If the user selects FSCRYPT_MODE_AES_128_CBC for the contents mode, an
ESSIV layer is automatically included.  In this case, before the IV is
passed to AES-128-CBC, it is encrypted with AES-256 where the AES-256
key is the SHA-256 hash of the file's contents encryption key.

Filenames encryption
--------------------

For filenames, each full filename is encrypted at once.  Because of
the requirements to retain support for efficient directory lookups and
filenames of up to 255 bytes, the same IV is used for every filename
in a directory.

However, each encrypted directory still uses a unique key, or
alternatively has the file's nonce (for `DIRECT_KEY policies`_) or
inode number (for `IV_INO_LBLK_64 policies`_) included in the IVs.
Thus, IV reuse is limited to within a single directory.

With CTS-CBC, the IV reuse means that when the plaintext filenames share a
common prefix at least as long as the cipher block size (16 bytes for AES), the
corresponding encrypted filenames will also share a common prefix.  This is
undesirable.  Adiantum and HCTR2 do not have this weakness, as they are
wide-block encryption modes.

All supported filenames encryption modes accept any plaintext length
>= 16 bytes; cipher block alignment is not required.  However,
filenames shorter than 16 bytes are NUL-padded to 16 bytes before
being encrypted.  In addition, to reduce leakage of filename lengths
via their ciphertexts, all filenames are NUL-padded to the next 4, 8,
16, or 32-byte boundary (configurable).  32 is recommended since this
provides the best confidentiality, at the cost of making directory
entries consume slightly more space.  Note that since NUL (``\0``) is
not otherwise a valid character in filenames, the padding will never
produce duplicate plaintexts.

Symbolic link targets are considered a type of filename and are
encrypted in the same way as filenames in directory entries, except
that IV reuse is not a problem as each symlink has its own inode.

User API
========

Setting an encryption policy
----------------------------

FS_IOC_SET_ENCRYPTION_POLICY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The FS_IOC_SET_ENCRYPTION_POLICY ioctl sets an encryption policy on an
empty directory or verifies that a directory or regular file already
has the specified encryption policy.  It takes in a pointer to
struct fscrypt_policy_v1 or struct fscrypt_policy_v2, defined as
follows::

    #define FSCRYPT_POLICY_V1               0
    #define FSCRYPT_KEY_DESCRIPTOR_SIZE     8
    struct fscrypt_policy_v1 {
            __u8 version;
            __u8 contents_encryption_mode;
            __u8 filenames_encryption_mode;
            __u8 flags;
            __u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
    };
    #define fscrypt_policy  fscrypt_policy_v1

    #define FSCRYPT_POLICY_V2               2
    #define FSCRYPT_KEY_IDENTIFIER_SIZE     16
    struct fscrypt_policy_v2 {
            __u8 version;
            __u8 contents_encryption_mode;
            __u8 filenames_encryption_mode;
            __u8 flags;
            __u8 log2_data_unit_size;
            __u8 __reserved[3];
            __u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
    };

This structure must be initialized as follows:

- ``version`` must be FSCRYPT_POLICY_V1 (0) if
  struct fscrypt_policy_v1 is used or FSCRYPT_POLICY_V2 (2) if
  struct fscrypt_policy_v2 is used. (Note: we refer to the original
  policy version as "v1", though its version code is really 0.)
  For new encrypted directories, use v2 policies.

- ``contents_encryption_mode`` and ``filenames_encryption_mode`` must
  be set to constants from ``<linux/fscrypt.h>`` which identify the
  encryption modes to use.  If unsure, use FSCRYPT_MODE_AES_256_XTS
  (1) for ``contents_encryption_mode`` and FSCRYPT_MODE_AES_256_CTS
  (4) for ``filenames_encryption_mode``.

- ``flags`` contains optional flags from ``<linux/fscrypt.h>``:

  - FSCRYPT_POLICY_FLAGS_PAD_*: The amount of NUL padding to use when
    encrypting filenames.  If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32
    (0x3).
  - FSCRYPT_POLICY_FLAG_DIRECT_KEY: See `DIRECT_KEY policies`_.
  - FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64: See `IV_INO_LBLK_64
    policies`_.
  - FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32: See `IV_INO_LBLK_32
    policies`_.

  v1 encryption policies only support the PAD_* and DIRECT_KEY flags.
  The other flags are only supported by v2 encryption policies.

  The DIRECT_KEY, IV_INO_LBLK_64, and IV_INO_LBLK_32 flags are
  mutually exclusive.

- ``log2_data_unit_size`` is the log2 of the data unit size in bytes,
  or 0 to select the default data unit size.  The data unit size is
  the granularity of file contents encryption.  For example, setting
  ``log2_data_unit_size`` to 12 causes file contents be passed to the
  underlying encryption algorithm (such as AES-256-XTS) in 4096-byte
  data units, each with its own IV.

  Not all filesystems support setting ``log2_data_unit_size``.  ext4
  and f2fs support it since Linux v6.7.  On filesystems that support
  it, the supported nonzero values are 9 through the log2 of the
  filesystem block size, inclusively.  The default value of 0 selects
  the filesystem block size.

  The main use case for ``log2_data_unit_size`` is for selecting a
  data unit size smaller than the filesystem block size for
  compatibility with inline encryption hardware that only supports
  smaller data unit sizes.  ``/sys/block/$disk/queue/crypto/`` may be
  useful for checking which data unit sizes are supported by a
  particular system's inline encryption hardware.

  Leave this field zeroed unless you are certain you need it.  Using
  an unnecessarily small data unit size reduces performance.

- For v2 encryption policies, ``__reserved`` must be zeroed.

- For v1 encryption policies, ``master_key_descriptor`` specifies how
  to find the master key in a keyring; see `Adding keys`_.  It is up
  to userspace to choose a unique ``master_key_descriptor`` for each
  master key.  The e4crypt and fscrypt tools use the first 8 bytes of
  ``SHA-512(SHA-512(master_key))``, but this particular scheme is not
  required.  Also, the master key need not be in the keyring yet when
  FS_IOC_SET_ENCRYPTION_POLICY is executed.  However, it must be added
  before any files can be created in the encrypted directory.

  For v2 encryption policies, ``master_key_descriptor`` has been
  replaced with ``master_key_identifier``, which is longer and cannot
  be arbitrarily chosen.  Instead, the key must first be added using
  `FS_IOC_ADD_ENCRYPTION_KEY`_.  Then, the ``key_spec.u.identifier``
  the kernel returned in the struct fscrypt_add_key_arg must
  be used as the ``master_key_identifier`` in
  struct fscrypt_policy_v2.

If the file is not yet encrypted, then FS_IOC_SET_ENCRYPTION_POLICY
verifies that the file is an empty directory.  If so, the specified
encryption policy is assigned to the directory, turning it into an
encrypted directory.  After that, and after providing the
corresponding master key as described in `Adding keys`_, all regular
files, directories (recursively), and symlinks created in the
directory will be encrypted, inheriting the same encryption policy.
The filenames in the directory's entries will be encrypted as well.

Alternatively, if the file is already encrypted, then
FS_IOC_SET_ENCRYPTION_POLICY validates that the specified encryption
policy exactly matches the actual one.  If they match, then the ioctl
returns 0.  Otherwise, it fails with EEXIST.  This works on both
regular files and directories, including nonempty directories.

When a v2 encryption policy is assigned to a directory, it is also
required that either the specified key has been added by the current
user or that the caller has CAP_FOWNER in the initial user namespace.
(This is needed to prevent a user from encrypting their data with
another user's key.)  The key must remain added while
FS_IOC_SET_ENCRYPTION_POLICY is executing.  However, if the new
encrypted directory does not need to be accessed immediately, then the
key can be removed right away afterwards.

Note that the ext4 filesystem does not allow the root directory to be
encrypted, even if it is empty.  Users who want to encrypt an entire
filesystem with one key should consider using dm-crypt instead.

FS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors:

- ``EACCES``: the file is not owned by the process's uid, nor does the
  process have the CAP_FOWNER capability in a namespace with the file
  owner's uid mapped
- ``EEXIST``: the file is already encrypted with an encryption policy
  different from the one specified
- ``EINVAL``: an invalid encryption policy was specified (invalid
  version, mode(s), or flags; or reserved bits were set); or a v1
  encryption policy was specified but the directory has the casefold
  flag enabled (casefolding is incompatible with v1 policies).
- ``ENOKEY``: a v2 encryption policy was specified, but the key with
  the specified ``master_key_identifier`` has not been added, nor does
  the process have the CAP_FOWNER capability in the initial user
  namespace
- ``ENOTDIR``: the file is unencrypted and is a regular file, not a
  directory
- ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``EOPNOTSUPP``: the kernel was not configured with encryption
  support for filesystems, or the filesystem superblock has not
  had encryption enabled on it.  (For example, to use encryption on an
  ext4 filesystem, CONFIG_FS_ENCRYPTION must be enabled in the
  kernel config, and the superblock must have had the "encrypt"
  feature flag enabled using ``tune2fs -O encrypt`` or ``mkfs.ext4 -O
  encrypt``.)
- ``EPERM``: this directory may not be encrypted, e.g. because it is
  the root directory of an ext4 filesystem
- ``EROFS``: the filesystem is readonly

Getting an encryption policy
----------------------------

Two ioctls are available to get a file's encryption policy:

- `FS_IOC_GET_ENCRYPTION_POLICY_EX`_
- `FS_IOC_GET_ENCRYPTION_POLICY`_

The extended (_EX) version of the ioctl is more general and is
recommended to use when possible.  However, on older kernels only the
original ioctl is available.  Applications should try the extended
version, and if it fails with ENOTTY fall back to the original
version.

FS_IOC_GET_ENCRYPTION_POLICY_EX
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption
policy, if any, for a directory or regular file.  No additional
permissions are required beyond the ability to open the file.  It
takes in a pointer to struct fscrypt_get_policy_ex_arg,
defined as follows::

    struct fscrypt_get_policy_ex_arg {
            __u64 policy_size; /* input/output */
            union {
                    __u8 version;
                    struct fscrypt_policy_v1 v1;
                    struct fscrypt_policy_v2 v2;
            } policy; /* output */
    };

The caller must initialize ``policy_size`` to the size available for
the policy struct, i.e. ``sizeof(arg.policy)``.

On success, the policy struct is returned in ``policy``, and its
actual size is returned in ``policy_size``.  ``policy.version`` should
be checked to determine the version of policy returned.  Note that the
version code for the "v1" policy is actually 0 (FSCRYPT_POLICY_V1).

FS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors:

- ``EINVAL``: the file is encrypted, but it uses an unrecognized
  encryption policy version
- ``ENODATA``: the file is not encrypted
- ``ENOTTY``: this type of filesystem does not implement encryption,
  or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
  (try FS_IOC_GET_ENCRYPTION_POLICY instead)
- ``EOPNOTSUPP``: the kernel was not configured with encryption
  support for this filesystem, or the filesystem superblock has not
  had encryption enabled on it
- ``EOVERFLOW``: the file is encrypted and uses a recognized
  encryption policy version, but the policy struct does not fit into
  the provided buffer

Note: if you only need to know whether a file is encrypted or not, on
most filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl
and check for FS_ENCRYPT_FL, or to use the statx() system call and
check for STATX_ATTR_ENCRYPTED in stx_attributes.

FS_IOC_GET_ENCRYPTION_POLICY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the
encryption policy, if any, for a directory or regular file.  However,
unlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_,
FS_IOC_GET_ENCRYPTION_POLICY only supports the original policy
version.  It takes in a pointer directly to struct fscrypt_policy_v1
rather than struct fscrypt_get_policy_ex_arg.

The error codes for FS_IOC_GET_ENCRYPTION_POLICY are the same as those
for FS_IOC_GET_ENCRYPTION_POLICY_EX, except that
FS_IOC_GET_ENCRYPTION_POLICY also returns ``EINVAL`` if the file is
encrypted using a newer encryption policy version.

Getting the per-filesystem salt
-------------------------------

Some filesystems, such as ext4 and F2FS, also support the deprecated
ioctl FS_IOC_GET_ENCRYPTION_PWSALT.  This ioctl retrieves a randomly
generated 16-byte value stored in the filesystem superblock.  This
value is intended to used as a salt when deriving an encryption key
from a passphrase or other low-entropy user credential.

FS_IOC_GET_ENCRYPTION_PWSALT is deprecated.  Instead, prefer to
generate and manage any needed salt(s) in userspace.

Getting a file's encryption nonce
---------------------------------

Since Linux v5.7, the ioctl FS_IOC_GET_ENCRYPTION_NONCE is supported.
On encrypted files and directories it gets the inode's 16-byte nonce.
On unencrypted files and directories, it fails with ENODATA.

This ioctl can be useful for automated tests which verify that the
encryption is being done correctly.  It is not needed for normal use
of fscrypt.

Adding keys
-----------

FS_IOC_ADD_ENCRYPTION_KEY
~~~~~~~~~~~~~~~~~~~~~~~~~

The FS_IOC_ADD_ENCRYPTION_KEY ioctl adds a master encryption key to
the filesystem, making all files on the filesystem which were
encrypted using that key appear "unlocked", i.e. in plaintext form.
It can be executed on any file or directory on the target filesystem,
but using the filesystem's root directory is recommended.  It takes in
a pointer to struct fscrypt_add_key_arg, defined as follows::

    struct fscrypt_add_key_arg {
            struct fscrypt_key_specifier key_spec;
            __u32 raw_size;
            __u32 key_id;
            __u32 __reserved[8];
            __u8 raw[];
    };

    #define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR        1
    #define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER        2

    struct fscrypt_key_specifier {
            __u32 type;     /* one of FSCRYPT_KEY_SPEC_TYPE_* */
            __u32 __reserved;
            union {
                    __u8 __reserved[32]; /* reserve some extra space */
                    __u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
                    __u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
            } u;
    };

    struct fscrypt_provisioning_key_payload {
            __u32 type;
            __u32 __reserved;
            __u8 raw[];
    };

struct fscrypt_add_key_arg must be zeroed, then initialized
as follows:

- If the key is being added for use by v1 encryption policies, then
  ``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and
  ``key_spec.u.descriptor`` must contain the descriptor of the key
  being added, corresponding to the value in the
  ``master_key_descriptor`` field of struct fscrypt_policy_v1.
  To add this type of key, the calling process must have the
  CAP_SYS_ADMIN capability in the initial user namespace.

  Alternatively, if the key is being added for use by v2 encryption
  policies, then ``key_spec.type`` must contain
  FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is
  an *output* field which the kernel fills in with a cryptographic
  hash of the key.  To add this type of key, the calling process does
  not need any privileges.  However, the number of keys that can be
  added is limited by the user's quota for the keyrings service (see
  ``Documentation/security/keys/core.rst``).

- ``raw_size`` must be the size of the ``raw`` key provided, in bytes.
  Alternatively, if ``key_id`` is nonzero, this field must be 0, since
  in that case the size is implied by the specified Linux keyring key.

- ``key_id`` is 0 if the raw key is given directly in the ``raw``
  field.  Otherwise ``key_id`` is the ID of a Linux keyring key of
  type "fscrypt-provisioning" whose payload is
  struct fscrypt_provisioning_key_payload whose ``raw`` field contains
  the raw key and whose ``type`` field matches ``key_spec.type``.
  Since ``raw`` is variable-length, the total size of this key's
  payload must be ``sizeof(struct fscrypt_provisioning_key_payload)``
  plus the raw key size.  The process must have Search permission on
  this key.

  Most users should leave this 0 and specify the raw key directly.
  The support for specifying a Linux keyring key is intended mainly to
  allow re-adding keys after a filesystem is unmounted and re-mounted,
  without having to store the raw keys in userspace memory.

- ``raw`` is a variable-length field which must contain the actual
  key, ``raw_size`` bytes long.  Alternatively, if ``key_id`` is
  nonzero, then this field is unused.

For v2 policy keys, the kernel keeps track of which user (identified
by effective user ID) added the key, and only allows the key to be
removed by that user --- or by "root", if they use
`FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_.

However, if another user has added the key, it may be desirable to
prevent that other user from unexpectedly removing it.  Therefore,
FS_IOC_ADD_ENCRYPTION_KEY may also be used to add a v2 policy key
*again*, even if it's already added by other user(s).  In this case,
FS_IOC_ADD_ENCRYPTION_KEY will just install a claim to the key for the
current user, rather than actually add the key again (but the raw key
must still be provided, as a proof of knowledge).

FS_IOC_ADD_ENCRYPTION_KEY returns 0 if either the key or a claim to
the key was either added or already exists.

FS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors:

- ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the
  caller does not have the CAP_SYS_ADMIN capability in the initial
  user namespace; or the raw key was specified by Linux key ID but the
  process lacks Search permission on the key.
- ``EDQUOT``: the key quota for this user would be exceeded by adding
  the key
- ``EINVAL``: invalid key size or key specifier type, or reserved bits
  were set
- ``EKEYREJECTED``: the raw key was specified by Linux key ID, but the
  key has the wrong type
- ``ENOKEY``: the raw key was specified by Linux key ID, but no key
  exists with that ID
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``EOPNOTSUPP``: the kernel was not configured with encryption
  support for this filesystem, or the filesystem superblock has not
  had encryption enabled on it

Legacy method
~~~~~~~~~~~~~

For v1 encryption policies, a master encryption key can also be
provided by adding it to a process-subscribed keyring, e.g. to a
session keyring, or to a user keyring if the user keyring is linked
into the session keyring.

This method is deprecated (and not supported for v2 encryption
policies) for several reasons.  First, it cannot be used in
combination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_),
so for removing a key a workaround such as keyctl_unlink() in
combination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would
have to be used.  Second, it doesn't match the fact that the
locked/unlocked status of encrypted files (i.e. whether they appear to
be in plaintext form or in ciphertext form) is global.  This mismatch
has caused much confusion as well as real problems when processes
running under different UIDs, such as a ``sudo`` command, need to
access encrypted files.

Nevertheless, to add a key to one of the process-subscribed keyrings,
the add_key() system call can be used (see:
``Documentation/security/keys/core.rst``).  The key type must be
"logon"; keys of this type are kept in kernel memory and cannot be
read back by userspace.  The key description must be "fscrypt:"
followed by the 16-character lower case hex representation of the
``master_key_descriptor`` that was set in the encryption policy.  The
key payload must conform to the following structure::

    #define FSCRYPT_MAX_KEY_SIZE            64

    struct fscrypt_key {
            __u32 mode;
            __u8 raw[FSCRYPT_MAX_KEY_SIZE];
            __u32 size;
    };

``mode`` is ignored; just set it to 0.  The actual key is provided in
``raw`` with ``size`` indicating its size in bytes.  That is, the
bytes ``raw[0..size-1]`` (inclusive) are the actual key.

The key description prefix "fscrypt:" may alternatively be replaced
with a filesystem-specific prefix such as "ext4:".  However, the
filesystem-specific prefixes are deprecated and should not be used in
new programs.

Removing keys
-------------

Two ioctls are available for removing a key that was added by
`FS_IOC_ADD_ENCRYPTION_KEY`_:

- `FS_IOC_REMOVE_ENCRYPTION_KEY`_
- `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_

These two ioctls differ only in cases where v2 policy keys are added
or removed by non-root users.

These ioctls don't work on keys that were added via the legacy
process-subscribed keyrings mechanism.

Before using these ioctls, read the `Kernel memory compromise`_
section for a discussion of the security goals and limitations of
these ioctls.

FS_IOC_REMOVE_ENCRYPTION_KEY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master
encryption key from the filesystem, and possibly removes the key
itself.  It can be executed on any file or directory on the target
filesystem, but using the filesystem's root directory is recommended.
It takes in a pointer to struct fscrypt_remove_key_arg, defined
as follows::

    struct fscrypt_remove_key_arg {
            struct fscrypt_key_specifier key_spec;
    #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY      0x00000001
    #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS     0x00000002
            __u32 removal_status_flags;     /* output */
            __u32 __reserved[5];
    };

This structure must be zeroed, then initialized as follows:

- The key to remove is specified by ``key_spec``:

    - To remove a key used by v1 encryption policies, set
      ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
      in ``key_spec.u.descriptor``.  To remove this type of key, the
      calling process must have the CAP_SYS_ADMIN capability in the
      initial user namespace.

    - To remove a key used by v2 encryption policies, set
      ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
      in ``key_spec.u.identifier``.

For v2 policy keys, this ioctl is usable by non-root users.  However,
to make this possible, it actually just removes the current user's
claim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY.
Only after all claims are removed is the key really removed.

For example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000,
then the key will be "claimed" by uid 1000, and
FS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000.  Or, if
both uids 1000 and 2000 added the key, then for each uid
FS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim.  Only
once *both* are removed is the key really removed.  (Think of it like
unlinking a file that may have hard links.)

If FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also
try to "lock" all files that had been unlocked with the key.  It won't
lock files that are still in-use, so this ioctl is expected to be used
in cooperation with userspace ensuring that none of the files are
still open.  However, if necessary, this ioctl can be executed again
later to retry locking any remaining files.

FS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed
(but may still have files remaining to be locked), the user's claim to
the key was removed, or the key was already removed but had files
remaining to be the locked so the ioctl retried locking them.  In any
of these cases, ``removal_status_flags`` is filled in with the
following informational status flags:

- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s)
  are still in-use.  Not guaranteed to be set in the case where only
  the user's claim to the key was removed.
- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the
  user's claim to the key was removed, not the key itself

FS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors:

- ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type
  was specified, but the caller does not have the CAP_SYS_ADMIN
  capability in the initial user namespace
- ``EINVAL``: invalid key specifier type, or reserved bits were set
- ``ENOKEY``: the key object was not found at all, i.e. it was never
  added in the first place or was already fully removed including all
  files locked; or, the user does not have a claim to the key (but
  someone else does).
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``EOPNOTSUPP``: the kernel was not configured with encryption
  support for this filesystem, or the filesystem superblock has not
  had encryption enabled on it

FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as
`FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the
ALL_USERS version of the ioctl will remove all users' claims to the
key, not just the current user's.  I.e., the key itself will always be
removed, no matter how many users have added it.  This difference is
only meaningful if non-root users are adding and removing keys.

Because of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires
"root", namely the CAP_SYS_ADMIN capability in the initial user
namespace.  Otherwise it will fail with EACCES.

Getting key status
------------------

FS_IOC_GET_ENCRYPTION_KEY_STATUS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a
master encryption key.  It can be executed on any file or directory on
the target filesystem, but using the filesystem's root directory is
recommended.  It takes in a pointer to
struct fscrypt_get_key_status_arg, defined as follows::

    struct fscrypt_get_key_status_arg {
            /* input */
            struct fscrypt_key_specifier key_spec;
            __u32 __reserved[6];

            /* output */
    #define FSCRYPT_KEY_STATUS_ABSENT               1
    #define FSCRYPT_KEY_STATUS_PRESENT              2
    #define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3
            __u32 status;
    #define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF   0x00000001
            __u32 status_flags;
            __u32 user_count;
            __u32 __out_reserved[13];
    };

The caller must zero all input fields, then fill in ``key_spec``:

    - To get the status of a key for v1 encryption policies, set
      ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
      in ``key_spec.u.descriptor``.

    - To get the status of a key for v2 encryption policies, set
      ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
      in ``key_spec.u.identifier``.

On success, 0 is returned and the kernel fills in the output fields:

- ``status`` indicates whether the key is absent, present, or
  incompletely removed.  Incompletely removed means that the master
  secret has been removed, but some files are still in use; i.e.,
  `FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational
  status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY.

- ``status_flags`` can contain the following flags:

    - ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key
      has added by the current user.  This is only set for keys
      identified by ``identifier`` rather than by ``descriptor``.

- ``user_count`` specifies the number of users who have added the key.
  This is only set for keys identified by ``identifier`` rather than
  by ``descriptor``.

FS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors:

- ``EINVAL``: invalid key specifier type, or reserved bits were set
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``EOPNOTSUPP``: the kernel was not configured with encryption
  support for this filesystem, or the filesystem superblock has not
  had encryption enabled on it

Among other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful
for determining whether the key for a given encrypted directory needs
to be added before prompting the user for the passphrase needed to
derive the key.

FS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in
the filesystem-level keyring, i.e. the keyring managed by
`FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_.  It
cannot get the status of a key that has only been added for use by v1
encryption policies using the legacy mechanism involving
process-subscribed keyrings.

Access semantics
================

With the key
------------

With the encryption key, encrypted regular files, directories, and
symlinks behave very similarly to their unencrypted counterparts ---
after all, the encryption is intended to be transparent.  However,
astute users may notice some differences in behavior:

- Unencrypted files, or files encrypted with a different encryption
  policy (i.e. different key, modes, or flags), cannot be renamed or
  linked into an encrypted directory; see `Encryption policy
  enforcement`_.  Attempts to do so will fail with EXDEV.  However,
  encrypted files can be renamed within an encrypted directory, or
  into an unencrypted directory.

  Note: "moving" an unencrypted file into an encrypted directory, e.g.
  with the `mv` program, is implemented in userspace by a copy
  followed by a delete.  Be aware that the original unencrypted data
  may remain recoverable from free space on the disk; prefer to keep
  all files encrypted from the very beginning.  The `shred` program
  may be used to overwrite the source files but isn't guaranteed to be
  effective on all filesystems and storage devices.

- Direct I/O is supported on encrypted files only under some
  circumstances.  For details, see `Direct I/O support`_.

- The fallocate operations FALLOC_FL_COLLAPSE_RANGE and
  FALLOC_FL_INSERT_RANGE are not supported on encrypted files and will
  fail with EOPNOTSUPP.

- Online defragmentation of encrypted files is not supported.  The
  EXT4_IOC_MOVE_EXT and F2FS_IOC_MOVE_RANGE ioctls will fail with
  EOPNOTSUPP.

- The ext4 filesystem does not support data journaling with encrypted
  regular files.  It will fall back to ordered data mode instead.

- DAX (Direct Access) is not supported on encrypted files.

- The maximum length of an encrypted symlink is 2 bytes shorter than
  the maximum length of an unencrypted symlink.  For example, on an
  EXT4 filesystem with a 4K block size, unencrypted symlinks can be up
  to 4095 bytes long, while encrypted symlinks can only be up to 4093
  bytes long (both lengths excluding the terminating null).

Note that mmap *is* supported.  This is possible because the pagecache
for an encrypted file contains the plaintext, not the ciphertext.

Without the key
---------------

Some filesystem operations may be performed on encrypted regular
files, directories, and symlinks even before their encryption key has
been added, or after their encryption key has been removed:

- File metadata may be read, e.g. using stat().

- Directories may be listed, in which case the filenames will be
  listed in an encoded form derived from their ciphertext.  The
  current encoding algorithm is described in `Filename hashing and
  encoding`_.  The algorithm is subject to change, but it is
  guaranteed that the presented filenames will be no longer than
  NAME_MAX bytes, will not contain the ``/`` or ``\0`` characters, and
  will uniquely identify directory entries.

  The ``.`` and ``..`` directory entries are special.  They are always
  present and are not encrypted or encoded.

- Files may be deleted.  That is, nondirectory files may be deleted
  with unlink() as usual, and empty directories may be deleted with
  rmdir() as usual.  Therefore, ``rm`` and ``rm -r`` will work as
  expected.

- Symlink targets may be read and followed, but they will be presented
  in encrypted form, similar to filenames in directories.  Hence, they
  are unlikely to point to anywhere useful.

Without the key, regular files cannot be opened or truncated.
Attempts to do so will fail with ENOKEY.  This implies that any
regular file operations that require a file descriptor, such as
read(), write(), mmap(), fallocate(), and ioctl(), are also forbidden.

Also without the key, files of any type (including directories) cannot
be created or linked into an encrypted directory, nor can a name in an
encrypted directory be the source or target of a rename, nor can an
O_TMPFILE temporary file be created in an encrypted directory.  All
such operations will fail with ENOKEY.

It is not currently possible to backup and restore encrypted files
without the encryption key.  This would require special APIs which
have not yet been implemented.

Encryption policy enforcement
=============================

After an encryption policy has been set on a directory, all regular
files, directories, and symbolic links created in that directory
(recursively) will inherit that encryption policy.  Special files ---
that is, named pipes, device nodes, and UNIX domain sockets --- will
not be encrypted.

Except for those special files, it is forbidden to have unencrypted
files, or files encrypted with a different encryption policy, in an
encrypted directory tree.  Attempts to link or rename such a file into
an encrypted directory will fail with EXDEV.  This is also enforced
during ->lookup() to provide limited protection against offline
attacks that try to disable or downgrade encryption in known locations
where applications may later write sensitive data.  It is recommended
that systems implementing a form of "verified boot" take advantage of
this by validating all top-level encryption policies prior to access.

Inline encryption support
=========================

By default, fscrypt uses the kernel crypto API for all cryptographic
operations (other than HKDF, which fscrypt partially implements
itself).  The kernel crypto API supports hardware crypto accelerators,
but only ones that work in the traditional way where all inputs and
outputs (e.g. plaintexts and ciphertexts) are in memory.  fscrypt can
take advantage of such hardware, but the traditional acceleration
model isn't particularly efficient and fscrypt hasn't been optimized
for it.

Instead, many newer systems (especially mobile SoCs) have *inline
encryption hardware* that can encrypt/decrypt data while it is on its
way to/from the storage device.  Linux supports inline encryption
through a set of extensions to the block layer called *blk-crypto*.
blk-crypto allows filesystems to attach encryption contexts to bios
(I/O requests) to specify how the data will be encrypted or decrypted
in-line.  For more information about blk-crypto, see
:ref:`Documentation/block/inline-encryption.rst <inline_encryption>`.

On supported filesystems (currently ext4 and f2fs), fscrypt can use
blk-crypto instead of the kernel crypto API to encrypt/decrypt file
contents.  To enable this, set CONFIG_FS_ENCRYPTION_INLINE_CRYPT=y in
the kernel configuration, and specify the "inlinecrypt" mount option
when mounting the filesystem.

Note that the "inlinecrypt" mount option just specifies to use inline
encryption when possible; it doesn't force its use.  fscrypt will
still fall back to using the kernel crypto API on files where the
inline encryption hardware doesn't have the needed crypto capabilities
(e.g. support for the needed encryption algorithm and data unit size)
and where blk-crypto-fallback is unusable.  (For blk-crypto-fallback
to be usable, it must be enabled in the kernel configuration with
CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK=y.)

Currently fscrypt always uses the filesystem block size (which is
usually 4096 bytes) as the data unit size.  Therefore, it can only use
inline encryption hardware that supports that data unit size.

Inline encryption doesn't affect the ciphertext or other aspects of
the on-disk format, so users may freely switch back and forth between
using "inlinecrypt" and not using "inlinecrypt".

Direct I/O support
==================

For direct I/O on an encrypted file to work, the following conditions
must be met (in addition to the conditions for direct I/O on an
unencrypted file):

* The file must be using inline encryption.  Usually this means that
  the filesystem must be mounted with ``-o inlinecrypt`` and inline
  encryption hardware must be present.  However, a software fallback
  is also available.  For details, see `Inline encryption support`_.

* The I/O request must be fully aligned to the filesystem block size.
  This means that the file position the I/O is targeting, the lengths
  of all I/O segments, and the memory addresses of all I/O buffers
  must be multiples of this value.  Note that the filesystem block
  size may be greater than the logical block size of the block device.

If either of the above conditions is not met, then direct I/O on the
encrypted file will fall back to buffered I/O.

Implementation details
======================

Encryption context
------------------

An encryption policy is represented on-disk by
struct fscrypt_context_v1 or struct fscrypt_context_v2.  It is up to
individual filesystems to decide where to store it, but normally it
would be stored in a hidden extended attribute.  It should *not* be
exposed by the xattr-related system calls such as getxattr() and
setxattr() because of the special semantics of the encryption xattr.
(In particular, there would be much confusion if an encryption policy
were to be added to or removed from anything other than an empty
directory.)  These structs are defined as follows::

    #define FSCRYPT_FILE_NONCE_SIZE 16

    #define FSCRYPT_KEY_DESCRIPTOR_SIZE  8
    struct fscrypt_context_v1 {
            u8 version;
            u8 contents_encryption_mode;
            u8 filenames_encryption_mode;
            u8 flags;
            u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
            u8 nonce[FSCRYPT_FILE_NONCE_SIZE];
    };

    #define FSCRYPT_KEY_IDENTIFIER_SIZE  16
    struct fscrypt_context_v2 {
            u8 version;
            u8 contents_encryption_mode;
            u8 filenames_encryption_mode;
            u8 flags;
            u8 __reserved[4];
            u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
            u8 nonce[FSCRYPT_FILE_NONCE_SIZE];
    };

The context structs contain the same information as the corresponding
policy structs (see `Setting an encryption policy`_), except that the
context structs also contain a nonce.  The nonce is randomly generated
by the kernel and is used as KDF input or as a tweak to cause
different files to be encrypted differently; see `Per-file encryption
keys`_ and `DIRECT_KEY policies`_.

Data path changes
-----------------

When inline encryption is used, filesystems just need to associate
encryption contexts with bios to specify how the block layer or the
inline encryption hardware will encrypt/decrypt the file contents.

When inline encryption isn't used, filesystems must encrypt/decrypt
the file contents themselves, as described below:

For the read path (->read_folio()) of regular files, filesystems can
read the ciphertext into the page cache and decrypt it in-place.  The
folio lock must be held until decryption has finished, to prevent the
folio from becoming visible to userspace prematurely.

For the write path (->writepage()) of regular files, filesystems
cannot encrypt data in-place in the page cache, since the cached
plaintext must be preserved.  Instead, filesystems must encrypt into a
temporary buffer or "bounce page", then write out the temporary
buffer.  Some filesystems, such as UBIFS, already use temporary
buffers regardless of encryption.  Other filesystems, such as ext4 and
F2FS, have to allocate bounce pages specially for encryption.

Filename hashing and encoding
-----------------------------

Modern filesystems accelerate directory lookups by using indexed
directories.  An indexed directory is organized as a tree keyed by
filename hashes.  When a ->lookup() is requested, the filesystem
normally hashes the filename being looked up so that it can quickly
find the corresponding directory entry, if any.

With encryption, lookups must be supported and efficient both with and
without the encryption key.  Clearly, it would not work to hash the
plaintext filenames, since the plaintext filenames are unavailable
without the key.  (Hashing the plaintext filenames would also make it
impossible for the filesystem's fsck tool to optimize encrypted
directories.)  Instead, filesystems hash the ciphertext filenames,
i.e. the bytes actually stored on-disk in the directory entries.  When
asked to do a ->lookup() with the key, the filesystem just encrypts
the user-supplied name to get the ciphertext.

Lookups without the key are more complicated.  The raw ciphertext may
contain the ``\0`` and ``/`` characters, which are illegal in
filenames.  Therefore, readdir() must base64url-encode the ciphertext
for presentation.  For most filenames, this works fine; on ->lookup(),
the filesystem just base64url-decodes the user-supplied name to get
back to the raw ciphertext.

However, for very long filenames, base64url encoding would cause the
filename length to exceed NAME_MAX.  To prevent this, readdir()
actually presents long filenames in an abbreviated form which encodes
a strong "hash" of the ciphertext filename, along with the optional
filesystem-specific hash(es) needed for directory lookups.  This
allows the filesystem to still, with a high degree of confidence, map
the filename given in ->lookup() back to a particular directory entry
that was previously listed by readdir().  See
struct fscrypt_nokey_name in the source for more details.

Note that the precise way that filenames are presented to userspace
without the key is subject to change in the future.  It is only meant
as a way to temporarily present valid filenames so that commands like
``rm -r`` work as expected on encrypted directories.

Tests
=====

To test fscrypt, use xfstests, which is Linux's de facto standard
filesystem test suite.  First, run all the tests in the "encrypt"
group on the relevant filesystem(s).  One can also run the tests
with the 'inlinecrypt' mount option to test the implementation for
inline encryption support.  For example, to test ext4 and
f2fs encryption using `kvm-xfstests
<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::

    kvm-xfstests -c ext4,f2fs -g encrypt
    kvm-xfstests -c ext4,f2fs -g encrypt -m inlinecrypt

UBIFS encryption can also be tested this way, but it should be done in
a separate command, and it takes some time for kvm-xfstests to set up
emulated UBI volumes::

    kvm-xfstests -c ubifs -g encrypt

No tests should fail.  However, tests that use non-default encryption
modes (e.g. generic/549 and generic/550) will be skipped if the needed
algorithms were not built into the kernel's crypto API.  Also, tests
that access the raw block device (e.g. generic/399, generic/548,
generic/549, generic/550) will be skipped on UBIFS.

Besides running the "encrypt" group tests, for ext4 and f2fs it's also
possible to run most xfstests with the "test_dummy_encryption" mount
option.  This option causes all new files to be automatically
encrypted with a dummy key, without having to make any API calls.
This tests the encrypted I/O paths more thoroughly.  To do this with
kvm-xfstests, use the "encrypt" filesystem configuration::

    kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto
    kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto -m inlinecrypt

Because this runs many more tests than "-g encrypt" does, it takes
much longer to run; so also consider using `gce-xfstests
<https://github.com/tytso/xfstests-bld/blob/master/Documentation/gce-xfstests.md>`_
instead of kvm-xfstests::

    gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto
    gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto -m inlinecrypt