fscrypt: add the test dummy encryption key on-demand

When the key for an inode is not found but the inode is using the
test_dummy_encryption policy, automatically add the
test_dummy_encryption key to the filesystem keyring.  This eliminates
the need for all the individual filesystems to do this at mount time,
which is a bit tricky to clean up from on failure.

Note: this covers the call to fscrypt_find_master_key() from inode key
setup, but not from the fscrypt ioctls.  So, this isn't *exactly* the
same as the key being present from the very beginning.  I think we can
tolerate that, though, since the inode key setup caller is the only one
that actually matters in the context of test_dummy_encryption.

Signed-off-by: Eric Biggers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

fs/crypto/fscrypt_private.h

@@ -651,6 +651,7 @@ bool fscrypt_policies_equal(const union fscrypt_policy *policy1,
 			    const union fscrypt_policy *policy2);
 int fscrypt_policy_to_key_spec(const union fscrypt_policy *policy,
 			       struct fscrypt_key_specifier *key_spec);
+const union fscrypt_policy *fscrypt_get_dummy_policy(struct super_block *sb);
 bool fscrypt_supported_policy(const union fscrypt_policy *policy_u,
 			      const struct inode *inode);
 int fscrypt_policy_from_context(union fscrypt_policy *policy_u,

fs/crypto/keysetup.c

@@ -438,6 +438,7 @@ static int setup_file_encryption_key(struct fscrypt_info *ci,
 				     bool need_dirhash_key,
 				     struct fscrypt_master_key **mk_ret)
 {
+	struct super_block *sb = ci->ci_inode->i_sb;
 	struct fscrypt_key_specifier mk_spec;
 	struct fscrypt_master_key *mk;
 	int err;
@@ -450,8 +451,28 @@ static int setup_file_encryption_key(struct fscrypt_info *ci,
 	if (err)
 		return err;
 
-	mk = fscrypt_find_master_key(ci->ci_inode->i_sb, &mk_spec);
-	if (!mk) {
+	mk = fscrypt_find_master_key(sb, &mk_spec);
+	if (unlikely(!mk)) {
+		const union fscrypt_policy *dummy_policy =
+			fscrypt_get_dummy_policy(sb);
+
+		/*
+		 * Add the test_dummy_encryption key on-demand.  In principle,
+		 * it should be added at mount time.  Do it here instead so that
+		 * the individual filesystems don't need to worry about adding
+		 * this key at mount time and cleaning up on mount failure.
+		 */
+		if (dummy_policy &&
+		    fscrypt_policies_equal(dummy_policy, &ci->ci_policy)) {
+			struct fscrypt_dummy_policy tmp = { dummy_policy };
+
+			err = fscrypt_add_test_dummy_key(sb, &tmp);
+			if (err)
+				return err;
+			mk = fscrypt_find_master_key(sb, &mk_spec);
+		}
+	}
+	if (unlikely(!mk)) {
 		if (ci->ci_policy.version != FSCRYPT_POLICY_V1)
 			return -ENOKEY;
 

fs/crypto/policy.c

@@ -53,8 +53,7 @@ int fscrypt_policy_to_key_spec(const union fscrypt_policy *policy,
 	}
 }
 
-static const union fscrypt_policy *
-fscrypt_get_dummy_policy(struct super_block *sb)
+const union fscrypt_policy *fscrypt_get_dummy_policy(struct super_block *sb)
 {
 	if (!sb->s_cop->get_dummy_policy)
 		return NULL;