Metadata support for v2 encryption policies

Linux v5.4 and later supports v2 encryption policies.  These have
several advantages over v1 encryption policies:

- Their encryption keys can be added/removed to/from the filesystem by
  non-root users, thus gaining the benefits of the filesystem keyring
  while also retaining support for non-root use.

- They use a more standard, secure, and flexible key derivation
  function.  Because of this, some future kernel-level fscrypt features
  will be implemented for v2 policies only.

- They prevent a denial-of-service attack where a user could associate
  the wrong key with another user's encrypted files.

Prepare the fscrypt tool to support v2 encryption policies by:

- Adding a policy_version field to the EncryptionOptions, i.e. to the
  config file and to the policy metadata files.

- Using the kernel-specified algorithm to compute the key descriptor for
  v2 policies.

- Handling setting and getting v2 policies.

Actually adding/removing the keys for v2 policies to/from the kernel is
left for the next patch.

1 files changed, 2 insertions, 0 deletions

diff --git a/metadata/metadata.proto b/metadata/metadata.proto
index e682212..81b3bf9 100644
--- a/metadata/metadata.proto
+++ b/metadata/metadata.proto
@@ -77,6 +77,8 @@ message EncryptionOptions {
 
   Mode contents = 2;
   Mode filenames = 3;
+
+  int64 policy_version = 4;
 }
 
 message WrappedPolicyKey {