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>

fs/crypto/fscrypt_private.h

@@ -431,6 +431,9 @@ extern struct key *
 fscrypt_find_master_key(struct super_block *sb,
 			const struct fscrypt_key_specifier *mk_spec);
 
+extern int fscrypt_verify_key_added(struct super_block *sb,
+				    const u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]);
+
 extern int __init fscrypt_init_keyring(void);
 
 /* keysetup.c */

fs/crypto/keyring.c

@@ -562,6 +562,53 @@ int fscrypt_ioctl_add_key(struct file *filp, void __user *_uarg)
 }
 EXPORT_SYMBOL_GPL(fscrypt_ioctl_add_key);
 
+/*
+ * Verify that the current user has added a master key with the given identifier
+ * (returns -ENOKEY if not).  This is needed to prevent a user from encrypting
+ * their files using some other user's key which they don't actually know.
+ * Cryptographically this isn't much of a problem, but the semantics of this
+ * would be a bit weird, so it's best to just forbid it.
+ *
+ * The system administrator (CAP_FOWNER) can override this, which should be
+ * enough for any use cases where encryption policies are being set using keys
+ * that were chosen ahead of time but aren't available at the moment.
+ *
+ * Note that the key may have already removed by the time this returns, but
+ * that's okay; we just care whether the key was there at some point.
+ *
+ * Return: 0 if the key is added, -ENOKEY if it isn't, or another -errno code
+ */
+int fscrypt_verify_key_added(struct super_block *sb,
+			     const u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE])
+{
+	struct fscrypt_key_specifier mk_spec;
+	struct key *key, *mk_user;
+	struct fscrypt_master_key *mk;
+	int err;
+
+	mk_spec.type = FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER;
+	memcpy(mk_spec.u.identifier, identifier, FSCRYPT_KEY_IDENTIFIER_SIZE);
+
+	key = fscrypt_find_master_key(sb, &mk_spec);
+	if (IS_ERR(key)) {
+		err = PTR_ERR(key);
+		goto out;
+	}
+	mk = key->payload.data[0];
+	mk_user = find_master_key_user(mk);
+	if (IS_ERR(mk_user)) {
+		err = PTR_ERR(mk_user);
+	} else {
+		key_put(mk_user);
+		err = 0;
+	}
+	key_put(key);
+out:
+	if (err == -ENOKEY && capable(CAP_FOWNER))
+		err = 0;
+	return err;
+}
+
 /*
  * Try to evict the inode's dentries from the dentry cache.  If the inode is a
  * directory, then it can have at most one dentry; however, that dentry may be

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);