@@ -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
-------------------
@@ -1339,6 +1354,27 @@ 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
-----------------
Add a couple of sections to the fscrypt documentation about per-extent encryption. Signed-off-by: Josef Bacik <josef@toxicpanda.com> --- Documentation/filesystems/fscrypt.rst | 36 +++++++++++++++++++++++++++ 1 file changed, 36 insertions(+)