diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
index def8908dd7e9..3e96e151df35 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -1837,11 +1837,13 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 				cifs_put_tlink(tlink);
 				return rc;
 			}
-			if (mode_from_sid)
-				nsecdesclen +=
-					le16_to_cpu(dacl_ptr->num_aces) * sizeof(struct smb_ace);
-			else /* cifsacl */
-				nsecdesclen += le16_to_cpu(dacl_ptr->size);
+			/*
+			 * Worst case: every ACE is rewritten with a new SID of
+			 * SID_MAX_SUB_AUTHORITIES sub-auths -> sizeof(smb_ace) each,
+			 * plus the smb_acl header replace_sids_and_copy_aces() emits.
+			 */
+			nsecdesclen += sizeof(struct smb_acl) +
+				le16_to_cpu(dacl_ptr->num_aces) * sizeof(struct smb_ace);
 		}
 	}