Add a couple of sections to the fscrypt documentation about per-extent encryption. Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx> --- Documentation/filesystems/fscrypt.rst | 41 +++++++++++++++++++++++++++ 1 file changed, 41 insertions(+) diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst index 1b84f818e574..dc8bf04484ba 100644 --- a/Documentation/filesystems/fscrypt.rst +++ b/Documentation/filesystems/fscrypt.rst @@ -256,6 +256,21 @@ alternative master keys or to support rotating master keys. Instead, the master keys may be wrapped in userspace, e.g. as is done by the `fscrypt <https://github.com/google/fscrypt>`_ tool. +Per-extent encryption keys +-------------------------- + +For certain file systems, such as btrfs, it's desired to derive a +per-extent encryption key. This is to enable features such as snapshots +and reflink, where you could have different inodes pointing at the same +extent. When a new extent is created fscrypt randomly generates a +16-byte nonce and the file system stores it along side the extent. +Then, it uses a KDF (as described in `Key derivation function`_) to +derive the extent's key from the master key and nonce. + +Currently the inode's master key and encryption policy must match the +extent, so you cannot share extents between inodes that were encrypted +differently. + DIRECT_KEY policies ------------------- @@ -1394,6 +1409,27 @@ by the kernel and is used as KDF input or as a tweak to cause different files to be encrypted differently; see `Per-file encryption keys`_ and `DIRECT_KEY policies`_. +Extent encryption context +------------------------- + +The extent encryption context mirrors the important parts of the above +`Encryption context`_, with a few ommisions. The struct is defined as +follows:: + + struct fscrypt_extent_context { + u8 version; + u8 encryption_mode; + u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; + u8 nonce[FSCRYPT_FILE_NONCE_SIZE]; + }; + +Currently all fields much match the containing inode's encryption +context, with the exception of the nonce. + +Additionally extent encryption is only supported with +FSCRYPT_EXTENT_CONTEXT_V2 using the standard policy, all other policies +are disallowed. + Data path changes ----------------- @@ -1417,6 +1453,11 @@ 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. +Inline encryption is not optional for extent encryption based file +systems, the amount of objects required to be kept around is too much. +Inline encryption handles the object lifetime details which results in a +cleaner implementation. + Filename hashing and encoding ----------------------------- -- 2.41.0
