diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 80f145c..eeb6fd67 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -77,26 +77,22 @@ static int derive_key_aes(u8 deriving_key[FS_AES_128_ECB_KEY_SIZE],
 
 static int validate_user_key(struct fscrypt_info *crypt_info,
 			struct fscrypt_context *ctx, u8 *raw_key,
-			u8 *prefix, int prefix_size)
+			const char *prefix)
 {
-	u8 *full_key_descriptor;
+	char *description;
 	struct key *keyring_key;
 	struct fscrypt_key *master_key;
 	const struct user_key_payload *ukp;
-	int full_key_len = prefix_size + (FS_KEY_DESCRIPTOR_SIZE * 2) + 1;
 	int res;
 
-	full_key_descriptor = kmalloc(full_key_len, GFP_NOFS);
-	if (!full_key_descriptor)
+	description = kasprintf(GFP_NOFS, "%s%*phN", prefix,
+				FS_KEY_DESCRIPTOR_SIZE,
+				ctx->master_key_descriptor);
+	if (!description)
 		return -ENOMEM;
 
-	memcpy(full_key_descriptor, prefix, prefix_size);
-	sprintf(full_key_descriptor + prefix_size,
-			"%*phN", FS_KEY_DESCRIPTOR_SIZE,
-			ctx->master_key_descriptor);
-	full_key_descriptor[full_key_len - 1] = '\0';
-	keyring_key = request_key(&key_type_logon, full_key_descriptor, NULL);
-	kfree(full_key_descriptor);
+	keyring_key = request_key(&key_type_logon, description, NULL);
+	kfree(description);
 	if (IS_ERR(keyring_key))
 		return PTR_ERR(keyring_key);
 
@@ -251,15 +247,10 @@ int fscrypt_get_crypt_info(struct inode *inode)
 	if (!raw_key)
 		goto out;
 
-	res = validate_user_key(crypt_info, &ctx, raw_key,
-			FS_KEY_DESC_PREFIX, FS_KEY_DESC_PREFIX_SIZE);
+	res = validate_user_key(crypt_info, &ctx, raw_key, FS_KEY_DESC_PREFIX);
 	if (res && inode->i_sb->s_cop->key_prefix) {
-		u8 *prefix = NULL;
-		int prefix_size, res2;
-
-		prefix_size = inode->i_sb->s_cop->key_prefix(inode, &prefix);
-		res2 = validate_user_key(crypt_info, &ctx, raw_key,
-							prefix, prefix_size);
+		int res2 = validate_user_key(crypt_info, &ctx, raw_key,
+					     inode->i_sb->s_cop->key_prefix);
 		if (res2) {
 			if (res2 == -ENOKEY)
 				res = -ENOKEY;