fscrypt: require that key be added when setting a v2 encryption policy

By looking up the master keys in a filesystem-level keyring rather than
in the calling processes' key hierarchy, it becomes possible for a user
to set an encryption policy which refers to some key they don't actually
know, then encrypt their files using that key.  Cryptographically this
isn't much of a problem, but the semantics of this would be a bit weird.
Thus, enforce that a v2 encryption policy can only be set if the user
has previously added the key, or has capable(CAP_FOWNER).

We tolerate that this problem will continue to exist for v1 encryption
policies, however; there is no way around that.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 0141d338c1fd..4072ba644595 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -233,11 +233,13 @@ static int set_encryption_policy(struct inode *inode,
 {
 	union fscrypt_context ctx;
 	int ctxsize;
+	int err;
 
 	if (!fscrypt_supported_policy(policy, inode))
 		return -EINVAL;
 
-	if (policy->version == FSCRYPT_POLICY_V1) {
+	switch (policy->version) {
+	case FSCRYPT_POLICY_V1:
 		/*
 		 * The original encryption policy version provided no way of
 		 * verifying that the correct master key was supplied, which was
@@ -251,6 +253,16 @@ static int set_encryption_policy(struct inode *inode,
 		 */
 		pr_warn_once("%s (pid %d) is setting deprecated v1 encryption policy; recommend upgrading to v2.\n",
 			     current->comm, current->pid);
+		break;
+	case FSCRYPT_POLICY_V2:
+		err = fscrypt_verify_key_added(inode->i_sb,
+					       policy->v2.master_key_identifier);
+		if (err)
+			return err;
+		break;
+	default:
+		WARN_ON(1);
+		return -EINVAL;
 	}
 
 	ctxsize = fscrypt_new_context_from_policy(&ctx, policy);