diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index dbced2937ec895..1bdb4a3a9b9fd6 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -440,6 +440,9 @@ extern struct key *
 fscrypt_find_master_key(struct super_block *sb,
 			const struct fscrypt_key_specifier *mk_spec);
 
+int fscrypt_add_test_dummy_key(struct super_block *sb,
+			       struct fscrypt_key_specifier *key_spec);
+
 extern int fscrypt_verify_key_added(struct super_block *sb,
 				    const u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]);
 
diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c
index ab41b25d4fa1ba..c983ddfde8ad8d 100644
--- a/fs/crypto/keyring.c
+++ b/fs/crypto/keyring.c
@@ -20,6 +20,7 @@
 
 #include <crypto/skcipher.h>
 #include <linux/key-type.h>
+#include <linux/random.h>
 #include <linux/seq_file.h>
 
 #include "fscrypt_private.h"
@@ -424,9 +425,9 @@ static int add_existing_master_key(struct fscrypt_master_key *mk,
 	return 0;
 }
 
-static int add_master_key(struct super_block *sb,
-			  struct fscrypt_master_key_secret *secret,
-			  const struct fscrypt_key_specifier *mk_spec)
+static int do_add_master_key(struct super_block *sb,
+			     struct fscrypt_master_key_secret *secret,
+			     const struct fscrypt_key_specifier *mk_spec)
 {
 	static DEFINE_MUTEX(fscrypt_add_key_mutex);
 	struct key *key;
@@ -465,6 +466,35 @@ static int add_master_key(struct super_block *sb,
 	return err;
 }
 
+static int add_master_key(struct super_block *sb,
+			  struct fscrypt_master_key_secret *secret,
+			  struct fscrypt_key_specifier *key_spec)
+{
+	int err;
+
+	if (key_spec->type == FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER) {
+		err = fscrypt_init_hkdf(&secret->hkdf, secret->raw,
+					secret->size);
+		if (err)
+			return err;
+
+		/*
+		 * Now that the HKDF context is initialized, the raw key is no
+		 * longer needed.
+		 */
+		memzero_explicit(secret->raw, secret->size);
+
+		/* Calculate the key identifier */
+		err = fscrypt_hkdf_expand(&secret->hkdf,
+					  HKDF_CONTEXT_KEY_IDENTIFIER, NULL, 0,
+					  key_spec->u.identifier,
+					  FSCRYPT_KEY_IDENTIFIER_SIZE);
+		if (err)
+			return err;
+	}
+	return do_add_master_key(sb, secret, key_spec);
+}
+
 static int fscrypt_provisioning_key_preparse(struct key_preparsed_payload *prep)
 {
 	const struct fscrypt_provisioning_key_payload *payload = prep->data;
@@ -609,6 +639,15 @@ int fscrypt_ioctl_add_key(struct file *filp, void __user *_uarg)
 	if (memchr_inv(arg.__reserved, 0, sizeof(arg.__reserved)))
 		return -EINVAL;
 
+	/*
+	 * Only root can add keys that are identified by an arbitrary descriptor
+	 * rather than by a cryptographic hash --- since otherwise a malicious
+	 * user could add the wrong key.
+	 */
+	if (arg.key_spec.type == FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR &&
+	    !capable(CAP_SYS_ADMIN))
+		return -EACCES;
+
 	memset(&secret, 0, sizeof(secret));
 	if (arg.key_id) {
 		if (arg.raw_size != 0)
@@ -626,54 +665,46 @@ int fscrypt_ioctl_add_key(struct file *filp, void __user *_uarg)
 			goto out_wipe_secret;
 	}
 
-	switch (arg.key_spec.type) {
-	case FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR:
-		/*
-		 * Only root can add keys that are identified by an arbitrary
-		 * descriptor rather than by a cryptographic hash --- since
-		 * otherwise a malicious user could add the wrong key.
-		 */
-		err = -EACCES;
-		if (!capable(CAP_SYS_ADMIN))
-			goto out_wipe_secret;
-		break;
-	case FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER:
-		err = fscrypt_init_hkdf(&secret.hkdf, secret.raw, secret.size);
-		if (err)
-			goto out_wipe_secret;
-
-		/*
-		 * Now that the HKDF context is initialized, the raw key is no
-		 * longer needed.
-		 */
-		memzero_explicit(secret.raw, secret.size);
-
-		/* Calculate the key identifier and return it to userspace. */
-		err = fscrypt_hkdf_expand(&secret.hkdf,
-					  HKDF_CONTEXT_KEY_IDENTIFIER,
-					  NULL, 0, arg.key_spec.u.identifier,
-					  FSCRYPT_KEY_IDENTIFIER_SIZE);
-		if (err)
-			goto out_wipe_secret;
-		err = -EFAULT;
-		if (copy_to_user(uarg->key_spec.u.identifier,
-				 arg.key_spec.u.identifier,
-				 FSCRYPT_KEY_IDENTIFIER_SIZE))
-			goto out_wipe_secret;
-		break;
-	default:
-		WARN_ON(1);
-		err = -EINVAL;
+	err = add_master_key(sb, &secret, &arg.key_spec);
+	if (err)
 		goto out_wipe_secret;
-	}
 
-	err = add_master_key(sb, &secret, &arg.key_spec);
+	/* Return the key identifier to userspace, if applicable */
+	err = -EFAULT;
+	if (arg.key_spec.type == FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER &&
+	    copy_to_user(uarg->key_spec.u.identifier, arg.key_spec.u.identifier,
+			 FSCRYPT_KEY_IDENTIFIER_SIZE))
+		goto out_wipe_secret;
+	err = 0;
 out_wipe_secret:
 	wipe_master_key_secret(&secret);
 	return err;
 }
 EXPORT_SYMBOL_GPL(fscrypt_ioctl_add_key);
 
+/*
+ * Add the key for '-o test_dummy_encryption' to the filesystem keyring.
+ *
+ * Use a per-boot random key to prevent people from misusing this option.
+ */
+int fscrypt_add_test_dummy_key(struct super_block *sb,
+			       struct fscrypt_key_specifier *key_spec)
+{
+	static u8 test_key[FSCRYPT_MAX_KEY_SIZE];
+	struct fscrypt_master_key_secret secret;
+	int err;
+
+	get_random_once(test_key, FSCRYPT_MAX_KEY_SIZE);
+
+	memset(&secret, 0, sizeof(secret));
+	secret.size = FSCRYPT_MAX_KEY_SIZE;
+	memcpy(secret.raw, test_key, FSCRYPT_MAX_KEY_SIZE);
+
+	err = add_master_key(sb, &secret, key_spec);
+	wipe_master_key_secret(&secret);
+	return err;
+}
+
 /*
  * 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
