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fscrypt: document the new ioctls and policy version

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Update the fscrypt documentation file to catch up to all the latest
changes, including the new ioctls to manage master encryption keys in
the filesystem-level keyring and the support for v2 encryption policies.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
This commit is contained in:
Eric Biggers 2019-08-04 19:35:49 -07:00
parent 62de25927a
commit ba13f2c8d7
1 changed files with 610 additions and 113 deletions
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723
Documentation/filesystems/fscrypt.rst
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@ -72,6 +72,9 @@ 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 are. If a vulnerable algorithm is used,
@ -80,29 +83,90 @@ 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.
Unauthorized file access
~~~~~~~~~~~~~~~~~~~~~~~~
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.)
After an encryption key has been added, fscrypt does not hide the
plaintext file contents or filenames from other users on the same
system. Instead, existing access control mechanisms such as file mode
bits, POSIX ACLs, LSMs, or namespaces should be used for this purpose.
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.
(For the reasoning behind this, understand that while the key is
added, the confidentiality of the data, from the perspective of the
system itself, is *not* protected by the mathematical properties of
encryption but rather only by the correctness of the kernel.
Therefore, any encryption-specific access control checks would merely
be enforced by kernel *code* and therefore would be largely redundant
with the wide variety of access control mechanisms already available.)
Kernel memory compromise
~~~~~~~~~~~~~~~~~~~~~~~~
An attacker who compromises the system enough to read from arbitrary
memory, e.g. by mounting a physical attack or by exploiting a kernel
security vulnerability, can compromise all encryption keys that are
currently in use.
However, fscrypt allows encryption keys to be removed from the kernel,
which may protect them from later compromise.
In more detail, the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl (or the
FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl) can wipe a master
encryption key from kernel memory. If it does so, it will also try to
evict all cached inodes which had been "unlocked" using the key,
thereby wiping their per-file keys and making them once again appear
"locked", i.e. in ciphertext or encrypted form.
However, these ioctls have some limitations:
- Per-file keys for in-use files will *not* be removed or wiped.
Therefore, for maximum effect, userspace should close the relevant
encrypted files and directories before removing a master key, as
well as kill any processes whose working directory is in an affected
encrypted directory.
- The kernel cannot magically wipe copies of the master key(s) that
userspace might have as well. Therefore, userspace must wipe all
copies of the master key(s) it makes as well; normally this should
be done immediately after FS_IOC_ADD_ENCRYPTION_KEY, without waiting
for FS_IOC_REMOVE_ENCRYPTION_KEY. Naturally, the same also applies
to all higher levels in the key hierarchy. Userspace should also
follow other security precautions such as mlock()ing memory
containing keys to prevent it from being swapped out.
- In general, decrypted contents and filenames in the kernel VFS
caches are freed but not wiped. Therefore, portions thereof may be
recoverable from freed memory, even after the corresponding key(s)
were wiped. To partially solve this, you can set
CONFIG_PAGE_POISONING=y in your kernel config and add page_poison=1
to your kernel command line. However, this has a performance cost.
- Secret keys might still exist in CPU registers, in crypto
accelerator hardware (if used by the crypto API to implement any of
the algorithms), or in other places not explicitly considered here.
Limitations of v1 policies
~~~~~~~~~~~~~~~~~~~~~~~~~~
v1 encryption policies have some weaknesses with respect to online
attacks:
- There is no verification that the provided master key is correct.
Therefore, a malicious user can temporarily associate the wrong key
with another user's encrypted files to which they have read-only
access. Because of filesystem caching, the wrong key will then be
used by the other user's accesses to those files, even if the other
user has the correct key in their own keyring. This violates the
meaning of "read-only access".
- A compromise of a per-file key also compromises the master key from
which it was derived.
- Non-root users cannot securely remove encryption keys.
All the above problems are fixed with v2 encryption policies. For
this reason among others, it is recommended to use v2 encryption
policies on all new encrypted directories.
Key hierarchy
=============
@ -123,11 +187,52 @@ 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.
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 keys
-------------
@ -138,29 +243,9 @@ 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 (Key Derivation
Function) to derive the file's key from the master key and nonce.
The Adiantum encryption mode (see `Encryption modes and usage`_) is
special, since it accepts longer IVs and is suitable for both contents
and filenames encryption. For it, a "direct key" option is offered
where the file's nonce is included in the IVs and the master key is
used for encryption directly. This improves performance; however,
users must not use the same master key for any other encryption mode.
Below, the KDF and design considerations are described in more detail.
The current KDF works by encrypting the master key with AES-128-ECB,
using the file's nonce as the AES key. The output is used as the
derived key. If the output is longer than needed, then it is
truncated to the needed length.
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.
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
@ -176,6 +261,37 @@ rejected as it would have prevented ext4 filesystems from being
resized, and by itself still wouldn't have been sufficient to prevent
the same key from being directly reused for both XTS and CTS-CBC.
DIRECT_KEY and per-mode keys
----------------------------
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 logical block number
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 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.
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.
Encryption modes and usage
==========================
@ -270,24 +386,44 @@ 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 a
:c:type:`struct fscrypt_policy`, defined as follows::
:c:type:`struct fscrypt_policy_v1` or a :c:type:`struct
fscrypt_policy_v2`, defined as follows::
#define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
struct fscrypt_policy {
#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 __reserved[4];
__u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
};
This structure must be initialized as follows:
- ``version`` must be 0.
- ``version`` must be FSCRYPT_POLICY_V1 (0) if the struct is
:c:type:`fscrypt_policy_v1` or FSCRYPT_POLICY_V2 (2) if the struct
is :c:type:`fscrypt_policy_v2`. (Note: we refer to the original
policy version as "v1", though its version code is really 0.) For
new encrypted directories, use v2 policies.
- ``contents_encryption_mode`` and ``filenames_encryption_mode`` must
be set to constants from ``<linux/fscrypt.h>`` which identify the
@ -297,21 +433,30 @@ This structure must be initialized as follows:
- ``flags`` must contain a value from ``<linux/fscrypt.h>`` which
identifies the amount of NUL-padding to use when encrypting
filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32 (0x3). In
addition, if the chosen encryption modes are both
filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32 (0x3).
Additionally, if the encryption modes are both
FSCRYPT_MODE_ADIANTUM, this can contain
FSCRYPT_POLICY_FLAG_DIRECT_KEY to specify that the master key should
be used directly, without key derivation.
FSCRYPT_POLICY_FLAG_DIRECT_KEY; see `DIRECT_KEY and per-mode keys`_.
- ``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
- For v2 encryption policies, ``__reserved`` must be zeroed.
- For v1 encryption policies, ``master_key_descriptor`` specifies how
to find the master key in a keyring; see `Adding keys`_. It is up
to userspace to choose a unique ``master_key_descriptor`` for each
master key. The e4crypt and fscrypt tools use the first 8 bytes of
``SHA-512(SHA-512(master_key))``, but this particular scheme is not
required. Also, the master key need not be in the keyring yet when
FS_IOC_SET_ENCRYPTION_POLICY is executed. However, it must be added
before any files can be created in the encrypted directory.
For v2 encryption policies, ``master_key_descriptor`` has been
replaced with ``master_key_identifier``, which is longer and cannot
be arbitrarily chosen. Instead, the key must first be added using
`FS_IOC_ADD_ENCRYPTION_KEY`_. Then, the ``key_spec.u.identifier``
the kernel returned in the :c:type:`struct fscrypt_add_key_arg` must
be used as the ``master_key_identifier`` in the :c:type:`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
@ -327,6 +472,15 @@ 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.
@ -339,7 +493,11 @@ FS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors:
- ``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)
version, mode(s), or flags; or reserved bits were set)
- ``ENOKEY``: a v2 encryption policy was specified, but the key with
the specified ``master_key_identifier`` has not been added, nor does
the process have the CAP_FOWNER capability in the initial user
namespace
- ``ENOTDIR``: the file is unencrypted and is a regular file, not a
directory
- ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory
@ -358,25 +516,78 @@ FS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors:
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.
Two ioctls are available to get a file's encryption policy:
FS_IOC_GET_ENCRYPTION_POLICY can fail with the following errors:
- `FS_IOC_GET_ENCRYPTION_POLICY_EX`_
- `FS_IOC_GET_ENCRYPTION_POLICY`_
The extended (_EX) version of the ioctl is more general and is
recommended to use when possible. However, on older kernels only the
original ioctl is available. Applications should try the extended
version, and if it fails with ENOTTY fall back to the original
version.
FS_IOC_GET_ENCRYPTION_POLICY_EX
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption
policy, if any, for a directory or regular file. No additional
permissions are required beyond the ability to open the file. It
takes in a pointer to a :c:type:`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 context format
encryption policy version
- ``ENODATA``: the file is not encrypted
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``ENOTTY``: this type of filesystem does not implement encryption,
or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
(try FS_IOC_GET_ENCRYPTION_POLICY instead)
- ``EOPNOTSUPP``: the kernel was not configured with encryption
support for this filesystem
- ``EOVERFLOW``: the file is encrypted and uses a recognized
encryption policy version, but the policy struct does not fit into
the provided buffer
Note: if you only need to know whether a file is encrypted or not, on
most filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl
and check for FS_ENCRYPT_FL, or to use the statx() system call and
check for STATX_ATTR_ENCRYPTED in stx_attributes.
FS_IOC_GET_ENCRYPTION_POLICY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the
encryption policy, if any, for a directory or regular file. However,
unlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_,
FS_IOC_GET_ENCRYPTION_POLICY only supports the original policy
version. It takes in a pointer directly to a :c:type:`struct
fscrypt_policy_v1` rather than a :c:type:`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
-------------------------------
@ -392,8 +603,115 @@ 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:
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 a :c:type:`struct fscrypt_add_key_arg`, defined as
follows::
struct fscrypt_add_key_arg {
struct fscrypt_key_specifier key_spec;
__u32 raw_size;
__u32 __reserved[9];
__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;
};
:c:type:`struct fscrypt_add_key_arg` must be zeroed, then initialized
as follows:
- If the key is being added for use by v1 encryption policies, then
``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and
``key_spec.u.descriptor`` must contain the descriptor of the key
being added, corresponding to the value in the
``master_key_descriptor`` field of :c:type:`struct
fscrypt_policy_v1`. To add this type of key, the calling process
must have the CAP_SYS_ADMIN capability in the initial user
namespace.
Alternatively, if the key is being added for use by v2 encryption
policies, then ``key_spec.type`` must contain
FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is
an *output* field which the kernel fills in with a cryptographic
hash of the key. To add this type of key, the calling process does
not need any privileges. However, the number of keys that can be
added is limited by the user's quota for the keyrings service (see
``Documentation/security/keys/core.rst``).
- ``raw_size`` must be the size of the ``raw`` key provided, in bytes.
- ``raw`` is a variable-length field which must contain the actual
key, ``raw_size`` bytes long.
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
- ``EDQUOT``: the key quota for this user would be exceeded by adding
the key
- ``EINVAL``: invalid key size or key specifier type, or reserved bits
were set
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``EOPNOTSUPP``: the kernel was not configured with encryption
support for this filesystem, or the filesystem superblock has not
had encryption enabled on it
Legacy method
~~~~~~~~~~~~~
For v1 encryption policies, a master encryption key can also be
provided by adding it to a process-subscribed keyring, e.g. to a
session keyring, or to a user keyring if the user keyring is linked
into the session keyring.
This method is deprecated (and not supported for v2 encryption
policies) for several reasons. First, it cannot be used in
combination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_),
so for removing a key a workaround such as keyctl_unlink() in
combination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would
have to be used. Second, it doesn't match the fact that the
locked/unlocked status of encrypted files (i.e. whether they appear to
be in plaintext form or in ciphertext form) is global. This mismatch
has caused much confusion as well as real problems when processes
running under different UIDs, such as a ``sudo`` command, need to
access encrypted files.
Nevertheless, to add a key to one of the process-subscribed keyrings,
the add_key() system call can be used (see:
``Documentation/security/keys/core.rst``). The key type must be
"logon"; keys of this type are kept in kernel memory and cannot be
read back by userspace. The key description must be "fscrypt:"
@ -401,12 +719,12 @@ 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
#define FSCRYPT_MAX_KEY_SIZE 64
struct fscrypt_key {
u32 mode;
u8 raw[FSCRYPT_MAX_KEY_SIZE];
u32 size;
__u32 mode;
__u8 raw[FSCRYPT_MAX_KEY_SIZE];
__u32 size;
};
``mode`` is ignored; just set it to 0. The actual key is provided in
@ -418,26 +736,194 @@ 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.
Removing keys
-------------
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.
Two ioctls are available for removing a key that was added by
`FS_IOC_ADD_ENCRYPTION_KEY`_:
- `FS_IOC_REMOVE_ENCRYPTION_KEY`_
- `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_
These two ioctls differ only in cases where v2 policy keys are added
or removed by non-root users.
These ioctls don't work on keys that were added via the legacy
process-subscribed keyrings mechanism.
Before using these ioctls, read the `Kernel memory compromise`_
section for a discussion of the security goals and limitations of
these ioctls.
FS_IOC_REMOVE_ENCRYPTION_KEY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master
encryption key from the filesystem, and possibly removes the key
itself. It can be executed on any file or directory on the target
filesystem, but using the filesystem's root directory is recommended.
It takes in a pointer to a :c:type:`struct fscrypt_remove_key_arg`,
defined as follows::
struct fscrypt_remove_key_arg {
struct fscrypt_key_specifier key_spec;
#define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY 0x00000001
#define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS 0x00000002
__u32 removal_status_flags; /* output */
__u32 __reserved[5];
};
This structure must be zeroed, then initialized as follows:
- The key to remove is specified by ``key_spec``:
- To remove a key used by v1 encryption policies, set
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
in ``key_spec.u.descriptor``. To remove this type of key, the
calling process must have the CAP_SYS_ADMIN capability in the
initial user namespace.
- To remove a key used by v2 encryption policies, set
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
in ``key_spec.u.identifier``.
For v2 policy keys, this ioctl is usable by non-root users. However,
to make this possible, it actually just removes the current user's
claim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY.
Only after all claims are removed is the key really removed.
For example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000,
then the key will be "claimed" by uid 1000, and
FS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000. Or, if
both uids 1000 and 2000 added the key, then for each uid
FS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim. Only
once *both* are removed is the key really removed. (Think of it like
unlinking a file that may have hard links.)
If FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also
try to "lock" all files that had been unlocked with the key. It won't
lock files that are still in-use, so this ioctl is expected to be used
in cooperation with userspace ensuring that none of the files are
still open. However, if necessary, this ioctl can be executed again
later to retry locking any remaining files.
FS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed
(but may still have files remaining to be locked), the user's claim to
the key was removed, or the key was already removed but had files
remaining to be the locked so the ioctl retried locking them. In any
of these cases, ``removal_status_flags`` is filled in with the
following informational status flags:
- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s)
are still in-use. Not guaranteed to be set in the case where only
the user's claim to the key was removed.
- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the
user's claim to the key was removed, not the key itself
FS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors:
- ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type
was specified, but the caller does not have the CAP_SYS_ADMIN
capability in the initial user namespace
- ``EINVAL``: invalid key specifier type, or reserved bits were set
- ``ENOKEY``: the key object was not found at all, i.e. it was never
added in the first place or was already fully removed including all
files locked; or, the user does not have a claim to the key (but
someone else does).
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``EOPNOTSUPP``: the kernel was not configured with encryption
support for this filesystem, or the filesystem superblock has not
had encryption enabled on it
FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as
`FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the
ALL_USERS version of the ioctl will remove all users' claims to the
key, not just the current user's. I.e., the key itself will always be
removed, no matter how many users have added it. This difference is
only meaningful if non-root users are adding and removing keys.
Because of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires
"root", namely the CAP_SYS_ADMIN capability in the initial user
namespace. Otherwise it will fail with EACCES.
Getting key status
------------------
FS_IOC_GET_ENCRYPTION_KEY_STATUS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a
master encryption key. It can be executed on any file or directory on
the target filesystem, but using the filesystem's root directory is
recommended. It takes in a pointer to a :c:type:`struct
fscrypt_get_key_status_arg`, defined as follows::
struct fscrypt_get_key_status_arg {
/* input */
struct fscrypt_key_specifier key_spec;
__u32 __reserved[6];
/* output */
#define FSCRYPT_KEY_STATUS_ABSENT 1
#define FSCRYPT_KEY_STATUS_PRESENT 2
#define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3
__u32 status;
#define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF 0x00000001
__u32 status_flags;
__u32 user_count;
__u32 __out_reserved[13];
};
The caller must zero all input fields, then fill in ``key_spec``:
- To get the status of a key for v1 encryption policies, set
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
in ``key_spec.u.descriptor``.
- To get the status of a key for v2 encryption policies, set
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
in ``key_spec.u.identifier``.
On success, 0 is returned and the kernel fills in the output fields:
- ``status`` indicates whether the key is absent, present, or
incompletely removed. Incompletely removed means that the master
secret has been removed, but some files are still in use; i.e.,
`FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational
status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY.
- ``status_flags`` can contain the following flags:
- ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key
has added by the current user. This is only set for keys
identified by ``identifier`` rather than by ``descriptor``.
- ``user_count`` specifies the number of users who have added the key.
This is only set for keys identified by ``identifier`` rather than
by ``descriptor``.
FS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors:
- ``EINVAL``: invalid key specifier type, or reserved bits were set
- ``ENOTTY``: this type of filesystem does not implement encryption
- ``EOPNOTSUPP``: the kernel was not configured with encryption
support for this filesystem, or the filesystem superblock has not
had encryption enabled on it
Among other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful
for determining whether the key for a given encrypted directory needs
to be added before prompting the user for the passphrase needed to
derive the key.
FS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in
the filesystem-level keyring, i.e. the keyring managed by
`FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_. It
cannot get the status of a key that has only been added for use by v1
encryption policies using the legacy mechanism involving
process-subscribed keyrings.
Access semantics
================
@ -500,7 +986,7 @@ Without the key
Some filesystem operations may be performed on encrypted regular
files, directories, and symlinks even before their encryption key has
been provided:
been added, or after their encryption key has been removed:
- File metadata may be read, e.g. using stat().
@ -565,20 +1051,20 @@ 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::
fscrypt_context_v1` or a :c:type:`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_KEY_DESCRIPTOR_SIZE 8
#define FS_KEY_DERIVATION_NONCE_SIZE 16
struct fscrypt_context {
u8 format;
#define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
struct fscrypt_context_v1 {
u8 version;
u8 contents_encryption_mode;
u8 filenames_encryption_mode;
u8 flags;
@ -586,12 +1072,23 @@ as follows::
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`_.
#define FSCRYPT_KEY_IDENTIFIER_SIZE 16
struct fscrypt_context_v2 {
u8 version;
u8 contents_encryption_mode;
u8 filenames_encryption_mode;
u8 flags;
u8 __reserved[4];
u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
u8 nonce[FS_KEY_DERIVATION_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 keys`_ and
`DIRECT_KEY and per-mode keys`_.
Data path changes
-----------------