Merge tag 'fscrypt-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/fscrypt

Pull fscrypt updates from Ted Ts'o:
 "Lots of cleanups, mostly courtesy by Eric Biggers"

* tag 'fscrypt-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/fscrypt:
  fscrypt: lock mutex before checking for bounce page pool
  fscrypt: add a documentation file for filesystem-level encryption
  ext4: switch to fscrypt_prepare_setattr()
  ext4: switch to fscrypt_prepare_lookup()
  ext4: switch to fscrypt_prepare_rename()
  ext4: switch to fscrypt_prepare_link()
  ext4: switch to fscrypt_file_open()
  fscrypt: new helper function - fscrypt_prepare_setattr()
  fscrypt: new helper function - fscrypt_prepare_lookup()
  fscrypt: new helper function - fscrypt_prepare_rename()
  fscrypt: new helper function - fscrypt_prepare_link()
  fscrypt: new helper function - fscrypt_file_open()
  fscrypt: new helper function - fscrypt_require_key()
  fscrypt: remove unneeded empty fscrypt_operations structs
  fscrypt: remove ->is_encrypted()
  fscrypt: switch from ->is_encrypted() to IS_ENCRYPTED()
  fs, fscrypt: add an S_ENCRYPTED inode flag
  fscrypt: clean up include file mess

2 files changed, 621 insertions, 0 deletions

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
new file mode 100644
index 000000000000..776ddc655f79
--- /dev/null
+++ b/Documentation/filesystems/fscrypt.rst
@@ -0,0 +1,610 @@
+=====================================
+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 EPERM.  However,
+  encrypted files can be renamed within an encrypted directory, or
+  into an unencrypted directory.
+
+- 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 may be slightly longer than
+  the plaintext due to the NUL-padding.
+
+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 EPERM.  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.
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 256e10eedba4..53b89d0edc15 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -315,3 +315,14 @@ exported for use by modules.
    :internal:
 
 .. kernel-doc:: fs/pipe.c
+
+Encryption API
+==============
+
+A library which filesystems can hook into to support transparent
+encryption of files and directories.
+
+.. toctree::
+    :maxdepth: 2
+
+    fscrypt