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. 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. Also, it is recommended to use
the fscrypt userspace tool, or other existing userspace tools such as
fscryptctl or Android’s key
management system, 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, UBIFS, and CephFS. 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.
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 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.
Supported modes¶
Currently, the following pairs of encryption modes are supported:
AES-256-XTS for contents and AES-256-CBC-CTS for filenames
AES-256-XTS for contents and AES-256-HCTR2 for filenames
Adiantum for both contents and filenames
AES-128-CBC-ESSIV for contents and AES-128-CBC-CTS for filenames
SM4-XTS for contents and SM4-CBC-CTS for filenames
Note: in the API, “CBC” means CBC-ESSIV, and “CTS” means CBC-CTS. So, for example, FSCRYPT_MODE_AES_256_CTS means AES-256-CBC-CTS.
Authenticated encryption modes are not currently supported because of the difficulty of dealing with ciphertext expansion. Therefore, contents encryption uses a block cipher in XTS mode or CBC-ESSIV mode, or a wide-block cipher. Filenames encryption uses a block cipher in CBC-CTS mode or a wide-block cipher.
The (AES-256-XTS, AES-256-CBC-CTS) pair is the recommended default. It is also the only option that is guaranteed to always be supported if the kernel supports fscrypt at all; see Kernel config options.
The (AES-256-XTS, AES-256-HCTR2) pair is also a good choice that upgrades the filenames encryption to use a wide-block cipher. (A wide-block cipher, also called a tweakable super-pseudorandom permutation, has the property that changing one bit scrambles the entire result.) As described in Filenames encryption, a wide-block cipher is the ideal mode for the problem domain, though CBC-CTS is the “least bad” choice among the alternatives. For more information about HCTR2, see the HCTR2 paper.
Adiantum is recommended on systems where AES is too slow due to lack of hardware acceleration for AES. Adiantum is a wide-block cipher that uses XChaCha12 and AES-256 as its underlying components. Most of the work is done by XChaCha12, which is much faster than AES when AES acceleration is unavailable. For more information about Adiantum, see the Adiantum paper.
The (AES-128-CBC-ESSIV, AES-128-CBC-CTS) pair exists only to support systems whose only form of AES acceleration is an off-CPU crypto accelerator such as CAAM or CESA that does not support XTS.
The remaining mode pairs are the “national pride ciphers”:
(SM4-XTS, SM4-CBC-CTS)
Generally speaking, these ciphers aren’t “bad” per se, but they receive limited security review compared to the usual choices such as AES and ChaCha. They also don’t bring much new to the table. It is suggested to only use these ciphers where their use is mandated.
Kernel config options¶
Enabling fscrypt support (CONFIG_FS_ENCRYPTION) automatically pulls in only the basic support from the crypto API needed to use AES-256-XTS and AES-256-CBC-CTS encryption. For optimal performance, it is strongly recommended to also enable any available platform-specific kconfig options that provide acceleration for the algorithm(s) you wish to use. Support for any “non-default” encryption modes typically requires extra kconfig options as well.
Below, some relevant options are listed by encryption mode. Note, acceleration options not listed below may be available for your platform; refer to the kconfig menus. File contents encryption can also be configured to use inline encryption hardware instead of the kernel crypto API (see Inline encryption support); in that case, the file contents mode doesn’t need to supported in the kernel crypto API, but the filenames mode still does.
- AES-256-XTS and AES-256-CBC-CTS
- Recommended:
arm64: CONFIG_CRYPTO_AES_ARM64_CE_BLK
x86: CONFIG_CRYPTO_AES_NI_INTEL
- AES-256-HCTR2
- Mandatory:
CONFIG_CRYPTO_HCTR2
- Recommended:
arm64: CONFIG_CRYPTO_AES_ARM64_CE_BLK
arm64: CONFIG_CRYPTO_POLYVAL_ARM64_CE
x86: CONFIG_CRYPTO_AES_NI_INTEL
x86: CONFIG_CRYPTO_POLYVAL_CLMUL_NI
- Adiantum
- Mandatory:
CONFIG_CRYPTO_ADIANTUM
- Recommended:
arm32: CONFIG_CRYPTO_CHACHA20_NEON
arm32: CONFIG_CRYPTO_NHPOLY1305_NEON
arm64: CONFIG_CRYPTO_CHACHA20_NEON
arm64: CONFIG_CRYPTO_NHPOLY1305_NEON
x86: CONFIG_CRYPTO_CHACHA20_X86_64
x86: CONFIG_CRYPTO_NHPOLY1305_SSE2
x86: CONFIG_CRYPTO_NHPOLY1305_AVX2
- AES-128-CBC-ESSIV and AES-128-CBC-CTS:
- Mandatory:
CONFIG_CRYPTO_ESSIV
CONFIG_CRYPTO_SHA256 or another SHA-256 implementation
- Recommended:
AES-CBC acceleration
fscrypt also uses HMAC-SHA512 for key derivation, so enabling SHA-512 acceleration is recommended:
- SHA-512
- Recommended:
arm64: CONFIG_CRYPTO_SHA512_ARM64_CE
x86: CONFIG_CRYPTO_SHA512_SSSE3
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_sizefield 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 CBC-CTS, 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:
versionmust 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_modeandfilenames_encryption_modemust be set to constants from<linux/fscrypt.h>which identify the encryption modes to use. If unsure, use FSCRYPT_MODE_AES_256_XTS (1) forcontents_encryption_modeand FSCRYPT_MODE_AES_256_CTS (4) forfilenames_encryption_mode. For details, see Encryption modes and usage.v1 encryption policies only support three combinations of modes: (FSCRYPT_MODE_AES_256_XTS, FSCRYPT_MODE_AES_256_CTS), (FSCRYPT_MODE_AES_128_CBC, FSCRYPT_MODE_AES_128_CTS), and (FSCRYPT_MODE_ADIANTUM, FSCRYPT_MODE_ADIANTUM). v2 policies support all combinations documented in Supported modes.
flagscontains 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_sizeis 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, settinglog2_data_unit_sizeto 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_sizeis 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,
__reservedmust be zeroed.For v1 encryption policies,
master_key_descriptorspecifies how to find the master key in a keyring; see Adding keys. It is up to userspace to choose a uniquemaster_key_descriptorfor each master key. The e4crypt and fscrypt tools use the first 8 bytes ofSHA-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_descriptorhas been replaced withmaster_key_identifier, which is longer and cannot be arbitrarily chosen. Instead, the key must first be added using FS_IOC_ADD_ENCRYPTION_KEY. Then, thekey_spec.u.identifierthe kernel returned in the struct fscrypt_add_key_arg must be used as themaster_key_identifierin 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 mappedEEXIST: the file is already encrypted with an encryption policy different from the one specifiedEINVAL: 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 specifiedmaster_key_identifierhas not been added, nor does the process have the CAP_FOWNER capability in the initial user namespaceENOTDIR: the file is unencrypted and is a regular file, not a directoryENOTEMPTY: the file is unencrypted and is a nonempty directoryENOTTY: this type of filesystem does not implement encryptionEOPNOTSUPP: 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 usingtune2fs -O encryptormkfs.ext4 -O encrypt.)EPERM: this directory may not be encrypted, e.g. because it is the root directory of an ext4 filesystemEROFS: the filesystem is readonly
Getting an encryption policy¶
Two ioctls are available to get a file’s 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 versionENODATA: the file is not encryptedENOTTY: 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 itEOVERFLOW: 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.typemust contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, andkey_spec.u.descriptormust contain the descriptor of the key being added, corresponding to the value in themaster_key_descriptorfield 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.typemust contain FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, andkey_spec.u.identifieris 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 (seeDocumentation/security/keys/core.rst).raw_sizemust be the size of therawkey provided, in bytes. Alternatively, ifkey_idis nonzero, this field must be 0, since in that case the size is implied by the specified Linux keyring key.key_idis 0 if the raw key is given directly in therawfield. Otherwisekey_idis the ID of a Linux keyring key of type “fscrypt-provisioning” whose payload is struct fscrypt_provisioning_key_payload whoserawfield contains the raw key and whosetypefield matcheskey_spec.type. Sincerawis variable-length, the total size of this key’s payload must besizeof(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.
rawis a variable-length field which must contain the actual key,raw_sizebytes long. Alternatively, ifkey_idis 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 keyEINVAL: invalid key size or key specifier type, or reserved bits were setEKEYREJECTED: the raw key was specified by Linux key ID, but the key has the wrong typeENOKEY: the raw key was specified by Linux key ID, but no key exists with that IDENOTTY: this type of filesystem does not implement encryptionEOPNOTSUPP: 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:
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.typeto FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill inkey_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.typeto FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill inkey_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 namespaceEINVAL: invalid key specifier type, or reserved bits were setENOKEY: 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 encryptionEOPNOTSUPP: 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.typeto FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill inkey_spec.u.descriptor.To get the status of a key for v2 encryption policies, set
key_spec.typeto FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill inkey_spec.u.identifier.
On success, 0 is returned and the kernel fills in the output fields:
statusindicates whether the key is absent, present, or incompletely removed. Incompletely removed means that removal has been initiated, 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_flagscan contain the following flags:FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELFindicates that the key has added by the current user. This is only set for keys identified byidentifierrather than bydescriptor.
user_countspecifies the number of users who have added the key. This is only set for keys identified byidentifierrather than bydescriptor.
FS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors:
EINVAL: invalid key specifier type, or reserved bits were setENOTTY: this type of filesystem does not implement encryptionEOPNOTSUPP: 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\0characters, 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,
rmandrm -rwill 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 Documentation/block/inline-encryption.rst.
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 inlinecryptand 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 log2_data_unit_size;
u8 __reserved[3];
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:
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 instead of kvm-xfstests:
gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto
gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto -m inlinecrypt