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diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst new file mode 100644 index 000000000..5b667ee12 --- /dev/null +++ b/Documentation/filesystems/fscrypt.rst @@ -0,0 +1,624 @@ +===================================== +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: + +fscrypt is only resistant to side-channel attacks, such as timing or +electromagnetic attacks, to the extent that the underlying Linux +Cryptographic API algorithms 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. + +After an encryption key has been provided, fscrypt is not designed to +hide the plaintext file contents or filenames from other users on the +same system, regardless of the visibility of the keyring key. +Instead, existing access control mechanisms such as file mode bits, +POSIX ACLs, LSMs, or mount namespaces should be used for this purpose. +Also note that as long as the encryption keys are *anywhere* in +memory, an online attacker can necessarily compromise them by mounting +a physical attack or by exploiting any kernel security vulnerability +which provides an arbitrary memory read primitive. + +While it is ostensibly possible to "evict" keys from the system, +recently accessed encrypted files will remain accessible at least +until the filesystem is unmounted or the VFS caches are dropped, e.g. +using ``echo 2 > /proc/sys/vm/drop_caches``. Even after that, if the +RAM is compromised before being powered off, it will likely still be +possible to recover portions of the plaintext file contents, if not +some of the encryption keys as well. (Since Linux v4.12, all +in-kernel keys related to fscrypt are sanitized before being freed. +However, userspace would need to do its part as well.) + +Currently, fscrypt does not prevent a user from maliciously providing +an incorrect key for another user's existing encrypted files. A +protection against this is planned. + +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 key length needed by the contents and filenames +encryption modes being used. For example, if AES-256-XTS is used for +contents encryption, the master key must be 64 bytes (512 bits). Note +that the XTS mode is defined to require a key twice as long as that +required by the underlying block cipher. + +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. + +Userspace should generate master keys either using a cryptographically +secure random number generator, or by using a KDF (Key Derivation +Function). Note that whenever a KDF is used to "stretch" a +lower-entropy secret such as a passphrase, it is critical that a KDF +designed for this purpose be used, such as scrypt, PBKDF2, or Argon2. + +Per-file keys +------------- + +Master keys are not used to encrypt file contents or names directly. +Instead, a unique key is derived for each encrypted file, including +each regular file, directory, and symbolic link. This has several +advantages: + +- In cryptosystems, the same key material should never be used for + different purposes. Using the master key as both an XTS key for + contents encryption and as a CTS-CBC key for filenames encryption + would violate this rule. +- Per-file keys simplify the choice of IVs (Initialization Vectors) + for contents encryption. Without per-file keys, to ensure IV + uniqueness both the inode and logical block number would need to be + encoded in the IVs. This would make it impossible to renumber + inodes, which e.g. ``resize2fs`` can do when resizing an ext4 + filesystem. With per-file keys, it is sufficient to encode just the + logical block number in the IVs. +- Per-file keys strengthen the encryption of filenames, where IVs are + reused out of necessity. With a unique key per directory, IV reuse + is limited to within a single directory. +- Per-file keys allow individual files to be securely erased simply by + securely erasing their keys. (Not yet implemented.) + +A KDF (Key Derivation Function) is used to derive per-file keys from +the master key. This is done instead of wrapping a randomly-generated +key for each file because it reduces the size of the encryption xattr, +which for some filesystems makes the xattr more likely to fit in-line +in the filesystem's inode table. With a KDF, only a 16-byte nonce is +required --- long enough to make key reuse extremely unlikely. A +wrapped key, on the other hand, would need to be up to 64 bytes --- +the length of an AES-256-XTS key. Furthermore, 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 done by the `fscrypt +<https://github.com/google/fscrypt>`_ tool. + +The current KDF encrypts the master key using the 16-byte nonce as an +AES-128-ECB key. The output is used as the derived key. If the +output is longer than needed, then it is truncated to the needed +length. Truncation is the norm for directories and symlinks, since +those use the CTS-CBC encryption mode which requires a key half as +long as that required by the XTS encryption mode. + +Note: this KDF meets the primary security requirement, which is to +produce unique derived keys that preserve the entropy of the master +key, assuming that the master key is already a good pseudorandom key. +However, it is nonstandard and has some problems such as being +reversible, so it is generally considered to be a mistake! It may be +replaced with HKDF or another more standard KDF in the future. + +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 + +It is strongly recommended to use AES-256-XTS for contents encryption. +AES-128-CBC was added only for low-powered embedded devices with +crypto accelerators such as CAAM or CESA that do not support XTS. + +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. + +For file contents, each filesystem block is encrypted independently. +Currently, only the case where the filesystem block size is equal to +the system's page size (usually 4096 bytes) is supported. With the +XTS mode of operation (recommended), the logical block number within +the file is used as the IV. With the CBC mode of operation (not +recommended), ESSIV is used; specifically, the IV for CBC is the +logical block number encrypted with AES-256, where the AES-256 key is +the SHA-256 hash of the inode's data encryption key. + +For filenames, the full filename is encrypted at once. Because of the +requirements to retain support for efficient directory lookups and +filenames of up to 255 bytes, a constant initialization vector (IV) is +used. However, each encrypted directory uses a unique key, which +limits IV reuse to within a single directory. Note that IV reuse in +the context of CTS-CBC encryption means that when the original +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; it may be fixed in +the future by switching to an encryption mode that is a strong +pseudorandom permutation on arbitrary-length messages, e.g. the HEH +(Hash-Encrypt-Hash) mode. + +Since filenames are encrypted with the CTS-CBC mode of operation, the +plaintext and ciphertext filenames need not be multiples of the AES +block size, i.e. 16 bytes. However, the minimum size that can be +encrypted is 16 bytes, so shorter filenames 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. Each +symlink also uses a unique key; hence, the hardcoded IV is not a +problem for symlinks. + +User API +======== + +Setting an 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 a +:c:type:`struct fscrypt_policy`, defined as follows:: + + #define FS_KEY_DESCRIPTOR_SIZE 8 + + struct fscrypt_policy { + __u8 version; + __u8 contents_encryption_mode; + __u8 filenames_encryption_mode; + __u8 flags; + __u8 master_key_descriptor[FS_KEY_DESCRIPTOR_SIZE]; + }; + +This structure must be initialized as follows: + +- ``version`` must be 0. + +- ``contents_encryption_mode`` and ``filenames_encryption_mode`` must + be set to constants from ``<linux/fs.h>`` which identify the + encryption modes to use. If unsure, use + FS_ENCRYPTION_MODE_AES_256_XTS (1) for ``contents_encryption_mode`` + and FS_ENCRYPTION_MODE_AES_256_CTS (4) for + ``filenames_encryption_mode``. + +- ``flags`` must be set to a value from ``<linux/fs.h>`` which + identifies the amount of NUL-padding to use when encrypting + filenames. If unsure, use FS_POLICY_FLAGS_PAD_32 (0x3). + +- ``master_key_descriptor`` specifies how to find the master key in + the 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. + +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. + +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) +- ``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 this filesystem, or the filesystem superblock has not + had encryption enabled on it. (For example, to use encryption on an + ext4 filesystem, CONFIG_EXT4_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 +---------------------------- + +The FS_IOC_GET_ENCRYPTION_POLICY ioctl retrieves the :c:type:`struct +fscrypt_policy`, if any, for a directory or regular file. See above +for the struct definition. No additional permissions are required +beyond the ability to open the file. + +FS_IOC_GET_ENCRYPTION_POLICY can fail with the following errors: + +- ``EINVAL``: the file is encrypted, but it uses an unrecognized + encryption context format +- ``ENODATA``: the file is not encrypted +- ``ENOTTY``: this type of filesystem does not implement encryption +- ``EOPNOTSUPP``: the kernel was not configured with encryption + support for this filesystem + +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. + +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. + +Adding keys +----------- + +To provide a master key, userspace must add it to an appropriate +keyring using the add_key() system call (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 FS_MAX_KEY_SIZE 64 + + struct fscrypt_key { + u32 mode; + u8 raw[FS_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. + +There are several different types of keyrings in which encryption keys +may be placed, such as a session keyring, a user session keyring, or a +user keyring. Each key must be placed in a keyring that is "attached" +to all processes that might need to access files encrypted with it, in +the sense that request_key() will find the key. Generally, if only +processes belonging to a specific user need to access a given +encrypted directory and no session keyring has been installed, then +that directory's key should be placed in that user's user session +keyring or user keyring. Otherwise, a session keyring should be +installed if needed, and the key should be linked into that session +keyring, or in a keyring linked into that session keyring. + +Note: introducing the complex visibility semantics of keyrings here +was arguably a mistake --- especially given that by design, after any +process successfully opens an encrypted file (thereby setting up the +per-file key), possessing the keyring key is not actually required for +any process to read/write the file until its in-memory inode is +evicted. In the future there probably should be a way to provide keys +directly to the filesystem instead, which would make the intended +semantics clearer. + +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 not supported on encrypted files. Attempts to use + direct I/O on such files will fall back to buffered I/O. + +- The fallocate operations FALLOC_FL_COLLAPSE_RANGE, + FALLOC_FL_INSERT_RANGE, and FALLOC_FL_ZERO_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 st_size of an encrypted symlink will not necessarily give the + length of the symlink target as required by POSIX. It will actually + give the length of the ciphertext, which will be slightly longer + than the plaintext due to NUL-padding and an extra 2-byte overhead. + +- 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 provided: + +- 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. + +Implementation details +====================== + +Encryption context +------------------ + +An encryption policy is represented on-disk by a :c:type:`struct +fscrypt_context`. 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.) The struct is defined +as follows:: + + #define FS_KEY_DESCRIPTOR_SIZE 8 + #define FS_KEY_DERIVATION_NONCE_SIZE 16 + + struct fscrypt_context { + u8 format; + u8 contents_encryption_mode; + u8 filenames_encryption_mode; + u8 flags; + u8 master_key_descriptor[FS_KEY_DESCRIPTOR_SIZE]; + u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE]; + }; + +Note that :c:type:`struct fscrypt_context` contains the same +information as :c:type:`struct fscrypt_policy` (see `Setting an +encryption policy`_), except that :c:type:`struct fscrypt_context` +also contains a nonce. The nonce is randomly generated by the kernel +and is used to derive the inode's encryption key as described in +`Per-file keys`_. + +Data path changes +----------------- + +For the read path (->readpage()) of regular files, filesystems can +read the ciphertext into the page cache and decrypt it in-place. The +page lock must be held until decryption has finished, to prevent the +page 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 base64-encode the ciphertext for +presentation. For most filenames, this works fine; on ->lookup(), the +filesystem just base64-decodes the user-supplied name to get back to +the raw ciphertext. + +However, for very long filenames, base64 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 :c:type:`struct +fscrypt_digested_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. |