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, 40 insertions, 1 deletions

diff --git a/metadata/config_test.go b/metadata/config_test.go
index 3c20c51..83c1eb0 100644
--- a/metadata/config_test.go
+++ b/metadata/config_test.go
@@ -48,7 +48,8 @@ var testConfigString = `{
 	"options": {
 		"padding": "32",
 		"contents": "AES_256_XTS",
-		"filenames": "AES_256_CTS"
+		"filenames": "AES_256_CTS",
+		"policy_version": "1"
 	},
 	"use_fs_keyring_for_v1_policies": false
 }
@@ -78,3 +79,41 @@ func TestRead(t *testing.T) {
 		t.Errorf("did not match: %s", testConfig)
 	}
 }
+
+// Makes sure we can parse a legacy config file that doesn't have the fields
+// that were added later.
+func TestOptionalFields(t *testing.T) {
+	contents := `{
+		"source": "custom_passphrase",
+		"hash_costs": {
+			"time": "10",
+			"memory": "4096",
+			"parallelism": "8"
+		},
+		"compatibility": "",
+		"options": {
+			"padding": "32",
+			"contents": "AES_256_XTS",
+			"filenames": "AES_256_CTS"
+		}
+	}
+	`
+	buf := bytes.NewBufferString(contents)
+	cfg, err := ReadConfig(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.GetUseFsKeyringForV1Policies() {
+		t.Error("use_fs_keyring_for_v1_policies should be false, but was true")
+	}
+	if cfg.Options.PolicyVersion != 0 {
+		t.Errorf("policy version should be 0, but was %d", cfg.Options.PolicyVersion)
+	}
+	if err = cfg.CheckValidity(); err != nil {
+		t.Error(err)
+	}
+	// CheckValidity() should change an unset policy version to 1.
+	if cfg.Options.PolicyVersion != 1 {
+		t.Errorf("policy version should be 1 now, but was %d", cfg.Options.PolicyVersion)
+	}
+}