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Diffstat (limited to 'Documentation/filesystems/fscrypt.rst')
| -rw-r--r-- | Documentation/filesystems/fscrypt.rst | 189 |
1 files changed, 150 insertions, 39 deletions
diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst index e80329908549..29e84d125e02 100644 --- a/Documentation/filesystems/fscrypt.rst +++ b/Documentation/filesystems/fscrypt.rst @@ -70,7 +70,7 @@ Online attacks -------------- fscrypt (and storage encryption in general) can only provide limited -protection, if any at all, against online attacks. In detail: +protection against online attacks. In detail: Side-channel attacks ~~~~~~~~~~~~~~~~~~~~ @@ -99,16 +99,23 @@ 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 -~~~~~~~~~~~~~~~~~~~~~~~~ +Read-only kernel memory compromise +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Unless `hardware-wrapped keys`_ are used, an attacker who gains the +ability to read from arbitrary kernel memory, e.g. by mounting a +physical attack or by exploiting a kernel security vulnerability, can +compromise all fscrypt keys that are currently in-use. This also +extends to cold boot attacks; if the system is suddenly powered off, +keys the system was using may remain in memory for a short time. -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, if hardware-wrapped keys are used, then the fscrypt master +keys and file contents encryption keys (but not other types of fscrypt +subkeys such as filenames encryption keys) are protected from +compromises of arbitrary kernel memory. -However, fscrypt allows encryption keys to be removed from the kernel, -which may protect them from later compromise. +In addition, 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 @@ -144,6 +151,24 @@ However, these ioctls have some limitations: accelerator hardware (if used by the crypto API to implement any of the algorithms), or in other places not explicitly considered here. +Full system compromise +~~~~~~~~~~~~~~~~~~~~~~ + +An attacker who gains "root" access and/or the ability to execute +arbitrary kernel code can freely exfiltrate data that is protected by +any in-use fscrypt keys. Thus, usually fscrypt provides no meaningful +protection in this scenario. (Data that is protected by a key that is +absent throughout the entire attack remains protected, modulo the +limitations of key removal mentioned above in the case where the key +was removed prior to the attack.) + +However, if `hardware-wrapped keys`_ are used, such attackers will be +unable to exfiltrate the master keys or file contents keys in a form +that will be usable after the system is powered off. This may be +useful if the attacker is significantly time-limited and/or +bandwidth-limited, so they can only exfiltrate some data and need to +rely on a later offline attack to exfiltrate the rest of it. + Limitations of v1 policies ~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -170,6 +195,10 @@ policies on all new encrypted directories. Key hierarchy ============= +Note: this section assumes the use of raw keys rather than +hardware-wrapped keys. The use of hardware-wrapped keys modifies the +key hierarchy slightly. For details, see `Hardware-wrapped keys`_. + Master Keys ----------- @@ -832,7 +861,9 @@ a pointer to struct fscrypt_add_key_arg, defined as follows:: struct fscrypt_key_specifier key_spec; __u32 raw_size; __u32 key_id; - __u32 __reserved[8]; + #define FSCRYPT_ADD_KEY_FLAG_HW_WRAPPED 0x00000001 + __u32 flags; + __u32 __reserved[7]; __u8 raw[]; }; @@ -851,7 +882,7 @@ a pointer to struct fscrypt_add_key_arg, defined as follows:: struct fscrypt_provisioning_key_payload { __u32 type; - __u32 __reserved; + __u32 flags; __u8 raw[]; }; @@ -879,24 +910,32 @@ as follows: Alternatively, if ``key_id`` is nonzero, this field must be 0, since in that case the size is implied by the specified Linux keyring key. -- ``key_id`` is 0 if the raw key is given directly in the ``raw`` - field. Otherwise ``key_id`` is the ID of a Linux keyring key of - type "fscrypt-provisioning" whose payload is - struct fscrypt_provisioning_key_payload whose ``raw`` field contains - the raw key and whose ``type`` field matches ``key_spec.type``. - Since ``raw`` is variable-length, the total size of this key's - payload must be ``sizeof(struct fscrypt_provisioning_key_payload)`` - plus the raw key size. The process must have Search permission on - this key. - - Most users should leave this 0 and specify the raw key directly. - The support for specifying a Linux keyring key is intended mainly to +- ``key_id`` is 0 if the key is given directly in the ``raw`` field. + Otherwise ``key_id`` is the ID of a Linux keyring key of type + "fscrypt-provisioning" whose payload is struct + fscrypt_provisioning_key_payload whose ``raw`` field contains the + key, whose ``type`` field matches ``key_spec.type``, and whose + ``flags`` field matches ``flags``. Since ``raw`` is + variable-length, the total size of this key's payload must be + ``sizeof(struct fscrypt_provisioning_key_payload)`` plus the number + of key bytes. The process must have Search permission on this key. + + Most users should leave this 0 and specify the key directly. The + support for specifying a Linux keyring key is intended mainly to allow re-adding keys after a filesystem is unmounted and re-mounted, - without having to store the raw keys in userspace memory. + without having to store the keys in userspace memory. + +- ``flags`` contains optional flags from ``<linux/fscrypt.h>``: + + - FSCRYPT_ADD_KEY_FLAG_HW_WRAPPED: This denotes that the key is a + hardware-wrapped key. See `Hardware-wrapped keys`_. This flag + can't be used if FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR is used. - ``raw`` is a variable-length field which must contain the actual key, ``raw_size`` bytes long. Alternatively, if ``key_id`` is - nonzero, then this field is unused. + nonzero, then this field is unused. Note that despite being named + ``raw``, if FSCRYPT_ADD_KEY_FLAG_HW_WRAPPED is specified then it + will contain a wrapped key, not a raw key. 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 @@ -908,8 +947,8 @@ 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). +current user, rather than actually add the key again (but the 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. @@ -918,20 +957,23 @@ FS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors: - ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the caller does not have the CAP_SYS_ADMIN capability in the initial - user namespace; or the raw key was specified by Linux key ID but the + user namespace; or the key was specified by Linux key ID but the process lacks Search permission on the key. +- ``EBADMSG``: invalid hardware-wrapped key - ``EDQUOT``: the key quota for this user would be exceeded by adding the key - ``EINVAL``: invalid key size or key specifier type, or reserved bits were set -- ``EKEYREJECTED``: the raw key was specified by Linux key ID, but the - key has the wrong type -- ``ENOKEY``: the raw key was specified by Linux key ID, but no key - exists with that ID +- ``EKEYREJECTED``: the key was specified by Linux key ID, but the key + has the wrong type +- ``ENOKEY``: the key was specified by Linux key ID, but no key exists + with that ID - ``ENOTTY``: this type of filesystem does not implement encryption - ``EOPNOTSUPP``: the kernel was not configured with encryption support for this filesystem, or the filesystem superblock has not - had encryption enabled on it + had encryption enabled on it; or a hardware wrapped key was specified + but the filesystem does not support inline encryption or the hardware + does not support hardware-wrapped keys Legacy method ~~~~~~~~~~~~~ @@ -994,9 +1036,8 @@ 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. +Before using these ioctls, read the `Online attacks`_ section for a +discussion of the security goals and limitations of these ioctls. FS_IOC_REMOVE_ENCRYPTION_KEY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -1316,7 +1357,8 @@ inline encryption hardware doesn't have the needed crypto capabilities (e.g. support for the needed encryption algorithm and data unit size) and where blk-crypto-fallback is unusable. (For blk-crypto-fallback to be usable, it must be enabled in the kernel configuration with -CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK=y.) +CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK=y, and the file must be +protected by a raw key rather than a hardware-wrapped key.) Currently fscrypt always uses the filesystem block size (which is usually 4096 bytes) as the data unit size. Therefore, it can only use @@ -1324,7 +1366,76 @@ inline encryption hardware that supports that data unit size. Inline encryption doesn't affect the ciphertext or other aspects of the on-disk format, so users may freely switch back and forth between -using "inlinecrypt" and not using "inlinecrypt". +using "inlinecrypt" and not using "inlinecrypt". An exception is that +files that are protected by a hardware-wrapped key can only be +encrypted/decrypted by the inline encryption hardware and therefore +can only be accessed when the "inlinecrypt" mount option is used. For +more information about hardware-wrapped keys, see below. + +Hardware-wrapped keys +--------------------- + +fscrypt supports using *hardware-wrapped keys* when the inline +encryption hardware supports it. Such keys are only present in kernel +memory in wrapped (encrypted) form; they can only be unwrapped +(decrypted) by the inline encryption hardware and are temporally bound +to the current boot. This prevents the keys from being compromised if +kernel memory is leaked. This is done without limiting the number of +keys that can be used and while still allowing the execution of +cryptographic tasks that are tied to the same key but can't use inline +encryption hardware, e.g. filenames encryption. + +Note that hardware-wrapped keys aren't specific to fscrypt; they are a +block layer feature (part of *blk-crypto*). For more details about +hardware-wrapped keys, see the block layer documentation at +:ref:`Documentation/block/inline-encryption.rst +<hardware_wrapped_keys>`. The rest of this section just focuses on +the details of how fscrypt can use hardware-wrapped keys. + +fscrypt supports hardware-wrapped keys by allowing the fscrypt master +keys to be hardware-wrapped keys as an alternative to raw keys. To +add a hardware-wrapped key with `FS_IOC_ADD_ENCRYPTION_KEY`_, +userspace must specify FSCRYPT_ADD_KEY_FLAG_HW_WRAPPED in the +``flags`` field of struct fscrypt_add_key_arg and also in the +``flags`` field of struct fscrypt_provisioning_key_payload when +applicable. The key must be in ephemerally-wrapped form, not +long-term wrapped form. + +Some limitations apply. First, files protected by a hardware-wrapped +key are tied to the system's inline encryption hardware. Therefore +they can only be accessed when the "inlinecrypt" mount option is used, +and they can't be included in portable filesystem images. Second, +currently the hardware-wrapped key support is only compatible with +`IV_INO_LBLK_64 policies`_ and `IV_INO_LBLK_32 policies`_, as it +assumes that there is just one file contents encryption key per +fscrypt master key rather than one per file. Future work may address +this limitation by passing per-file nonces down the storage stack to +allow the hardware to derive per-file keys. + +Implementation-wise, to encrypt/decrypt the contents of files that are +protected by a hardware-wrapped key, fscrypt uses blk-crypto, +attaching the hardware-wrapped key to the bio crypt contexts. As is +the case with raw keys, the block layer will program the key into a +keyslot when it isn't already in one. However, when programming a +hardware-wrapped key, the hardware doesn't program the given key +directly into a keyslot but rather unwraps it (using the hardware's +ephemeral wrapping key) and derives the inline encryption key from it. +The inline encryption key is the key that actually gets programmed +into a keyslot, and it is never exposed to software. + +However, fscrypt doesn't just do file contents encryption; it also +uses its master keys to derive filenames encryption keys, key +identifiers, and sometimes some more obscure types of subkeys such as +dirhash keys. So even with file contents encryption out of the +picture, fscrypt still needs a raw key to work with. To get such a +key from a hardware-wrapped key, fscrypt asks the inline encryption +hardware to derive a cryptographically isolated "software secret" from +the hardware-wrapped key. fscrypt uses this "software secret" to key +its KDF to derive all subkeys other than file contents keys. + +Note that this implies that the hardware-wrapped key feature only +protects the file contents encryption keys. It doesn't protect other +fscrypt subkeys such as filenames encryption keys. Direct I/O support ================== @@ -1409,7 +1520,7 @@ read the ciphertext into the page cache and decrypt it in-place. The folio lock must be held until decryption has finished, to prevent the folio from becoming visible to userspace prematurely. -For the write path (->writepage()) of regular files, filesystems +For the write path (->writepages()) 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 |