diff --git a/fs/ceph/crypto.c b/fs/ceph/crypto.c
index bc0a097a4cea..845b69ba890c 100644
--- a/fs/ceph/crypto.c
+++ b/fs/ceph/crypto.c
@@ -313,6 +313,7 @@ int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
 	struct fscrypt_str iname;
 	char *name = fname->name;
 	int name_len = fname->name_len;
+	int dst_len;
 	int ret;
 
 	if (WARN_ON_ONCE(tname && is_vmalloc_addr(tname)))
@@ -387,6 +388,9 @@ int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
 		iname.len = fname->ctext_len;
 	}
 
+	/* oname->len is overwritten by the decryption below */
+	dst_len = oname->len;
+
 	_oname.name = unlikely(is_vmalloc_addr(oname->name)) ? tname : oname->name;
 	_oname.len = oname->len;
 
@@ -403,6 +407,10 @@ int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
 
 		name_len = snprintf(tmp_buf, sizeof(tmp_buf), "_%.*s_%llu",
 				    oname->len, oname->name, dir->i_ino);
+		if (name_len > dst_len) {
+			ret = -ENAMETOOLONG;
+			goto out;
+		}
 		memcpy(oname->name, tmp_buf, name_len);
 		oname->len = name_len;
 	}