fscrypt: check for appropriate use of DIRECT_KEY flag earlier

FSCRYPT_POLICY_FLAG_DIRECT_KEY is currently only allowed with Adiantum
encryption.  But FS_IOC_SET_ENCRYPTION_POLICY allowed it in combination
with other encryption modes, and an error wasn't reported until later
when the encrypted directory was actually used.

Fix it to report the error earlier by validating the correct use of the
DIRECT_KEY flag in fscrypt_supported_policy(), similar to how we
validate the IV_INO_LBLK_64 flag.

Link: https://lore.kernel.org/r/20191209211829.239800-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index fdb13ce69cd2..e785b00f19b3 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -29,6 +29,26 @@ bool fscrypt_policies_equal(const union fscrypt_policy *policy1,
 	return !memcmp(policy1, policy2, fscrypt_policy_size(policy1));
 }
 
+static bool supported_direct_key_modes(const struct inode *inode,
+				       u32 contents_mode, u32 filenames_mode)
+{
+	const struct fscrypt_mode *mode;
+
+	if (contents_mode != filenames_mode) {
+		fscrypt_warn(inode,
+			     "Direct key flag not allowed with different contents and filenames modes");
+		return false;
+	}
+	mode = &fscrypt_modes[contents_mode];
+
+	if (mode->ivsize < offsetofend(union fscrypt_iv, nonce)) {
+		fscrypt_warn(inode, "Direct key flag not allowed with %s",
+			     mode->friendly_name);
+		return false;
+	}
+	return true;
+}
+
 static bool supported_iv_ino_lblk_64_policy(
 					const struct fscrypt_policy_v2 *policy,
 					const struct inode *inode)
@@ -82,6 +102,11 @@ static bool fscrypt_supported_v1_policy(const struct fscrypt_policy_v1 *policy,
 		return false;
 	}
 
+	if ((policy->flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) &&
+	    !supported_direct_key_modes(inode, policy->contents_encryption_mode,
+					policy->filenames_encryption_mode))
+		return false;
+
 	return true;
 }
 
@@ -103,6 +128,11 @@ static bool fscrypt_supported_v2_policy(const struct fscrypt_policy_v2 *policy,
 		return false;
 	}
 
+	if ((policy->flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) &&
+	    !supported_direct_key_modes(inode, policy->contents_encryption_mode,
+					policy->filenames_encryption_mode))
+		return false;
+
 	if ((policy->flags & FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64) &&
 	    !supported_iv_ino_lblk_64_policy(policy, inode))
 		return false;