From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 20929C433FE for ; Wed, 26 Oct 2022 15:02:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229949AbiJZPCW (ORCPT ); Wed, 26 Oct 2022 11:02:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45810 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234115AbiJZPCU (ORCPT ); Wed, 26 Oct 2022 11:02:20 -0400 Received: from netrider.rowland.org (netrider.rowland.org [192.131.102.5]) by lindbergh.monkeyblade.net (Postfix) with SMTP id AED12A50DA for ; Wed, 26 Oct 2022 08:02:18 -0700 (PDT) Received: (qmail 1489809 invoked by uid 1000); 26 Oct 2022 11:02:17 -0400 Date: Wed, 26 Oct 2022 11:02:17 -0400 From: Alan Stern To: Eli Billauer Cc: gregkh@linuxfoundation.org, arnd@arndb.de, linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org, imv4bel@gmail.com Subject: Re: [PATCH] char: xillybus: Prevent use-after-free due to race condition Message-ID: References: <2e5cbdfe-f6cd-d24f-9785-55176af6c975@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2e5cbdfe-f6cd-d24f-9785-55176af6c975@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 26, 2022 at 11:52:40AM +0300, Eli Billauer wrote: > xillybus_find_inode() is called by xillybus_open() and xillyusb_open() > to translate the inode's major and minor into a pointer to a relevant > data structure and an index. > > But with xillyusb_open(), the data structure can be freed by > xillyusb_disconnect() during an unintentional time gap between the > release of the mutex that is taken by xillybus_find_inode() and the > mutex that is subsequently taken by xillyusb_open(). > > To fix this, xillybus_find_inode() supplies the pointer to the mutex that > it has locked (when returning success), so xillyusb_open() releases this > mutex only after obtaining the mutex that is specific to a device file. > This ensures that xillyusb_disconnect() won't release anything that is in > use. The standard way of handling this problem is different from this. The driver defines a private mutex, and it ensures that any routine calling *_find_inode() holds the mutex. It also ensures that the mutex is held while a new device is being registered and while a device is being removed. Even that won't fix all the synchronization problems. A process can open a device, and then after the device has been removed the process can still try to access the device. The driver needs to ensure that such accesses are not allowed. Alan Stern