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 X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C818CC55189 for ; Wed, 22 Apr 2020 07:18:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A4AA7206E9 for ; Wed, 22 Apr 2020 07:18:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726324AbgDVHSO (ORCPT ); Wed, 22 Apr 2020 03:18:14 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:34930 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725786AbgDVHSO (ORCPT ); Wed, 22 Apr 2020 03:18:14 -0400 Received: (from willy@localhost) by pcw.home.local (8.15.2/8.15.2/Submit) id 03M7HuOx016837; Wed, 22 Apr 2020 09:17:56 +0200 Date: Wed, 22 Apr 2020 09:17:56 +0200 From: Willy Tarreau To: Christoph Hellwig Cc: Denis Efremov , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] floppy: suppress UBSAN warning in setup_rw_floppy() Message-ID: <20200422071756.GA16814@1wt.eu> References: <20200421125722.58959-1-efremov@linux.com> <20200421125722.58959-4-efremov@linux.com> <20200422070921.GA19116@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200422070921.GA19116@infradead.org> User-Agent: Mutt/1.6.1 (2016-04-27) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Apr 22, 2020 at 12:09:21AM -0700, Christoph Hellwig wrote: > On Tue, Apr 21, 2020 at 03:57:22PM +0300, Denis Efremov wrote: > > UBSAN: array-index-out-of-bounds in drivers/block/floppy.c:1521:45 > > index 16 is out of range for type 'unsigned char [16]' > > Call Trace: > > ... > > setup_rw_floppy+0x5c3/0x7f0 > > floppy_ready+0x2be/0x13b0 > > process_one_work+0x2c1/0x5d0 > > worker_thread+0x56/0x5e0 > > kthread+0x122/0x170 > > ret_from_fork+0x35/0x40 > > > > >From include/uapi/linux/fd.h: > > struct floppy_raw_cmd { > > ... > > unsigned char cmd_count; > > unsigned char cmd[16]; > > unsigned char reply_count; > > unsigned char reply[16]; > > ... > > } > > > > This out-of-bounds access is intentional. The command in struct > > floppy_raw_cmd may take up the space initially intended for the reply > > and the reply count. It is needed for long 82078 commands such as > > RESTORE, which takes 17 command bytes. Initial cmd size is not enough > > and since struct setup_rw_floppy is a part of uapi we check that > > cmd_count is in [0:16+1+16] in raw_cmd_copyin(). > > > > The patch replaces array subscript with pointer arithetic to suppress > > UBSAN warning. > > Urghh. I think the better way would be to use a union that creates > a larger cmd field, or something like: > > struct floppy_raw_cmd { > ... > u8 buf[34]; > > #define BUF_CMD_COUNT 0 > #define BUF_CMD 1 > #define BUF_REPLY_COUNT 17 > #define BUF_REPLY 18 > > and use addressing based on that. But isn't it a problem if struct floppy_raw_cmd is exposed to uapi ? That said I remember a discussion with Linus who said that most if not all of the floppy parts leaking to uapi were more of a side effect of the include files reordering than a deliberate decision to expose it. So maybe that could remain the best solution indeed. I must say I don't feel very comfortable either with replacing p[i] with *(p+i) given that they are all supposed to be interchangeable and equivalent (as well as i[p] as strange as it can look). So we're not really protected against a later mechanical change or cleanup that reintroduces it :-/ Willy