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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 44104C31E40 for ; Thu, 15 Aug 2019 11:33:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 21CBA2086C for ; Thu, 15 Aug 2019 11:33:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731089AbfHOLdn (ORCPT ); Thu, 15 Aug 2019 07:33:43 -0400 Received: from szxga06-in.huawei.com ([45.249.212.32]:57430 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726120AbfHOLdn (ORCPT ); Thu, 15 Aug 2019 07:33:43 -0400 Received: from DGGEMS402-HUB.china.huawei.com (unknown [172.30.72.59]) by Forcepoint Email with ESMTP id 22C3DE5DA40BEEDEF668; Thu, 15 Aug 2019 19:33:35 +0800 (CST) Received: from [127.0.0.1] (10.177.244.145) by DGGEMS402-HUB.china.huawei.com (10.3.19.202) with Microsoft SMTP Server id 14.3.439.0; Thu, 15 Aug 2019 19:33:30 +0800 Subject: Re: [PATCH v4] ext4: fix potential use after free in system zone via remount with noblock_validity To: Jan Kara References: <20190815081631.19437-1-yi.zhang@huawei.com> <20190815093542.GA14313@quack2.suse.cz> CC: , , From: "zhangyi (F)" Message-ID: <7a22edf0-9c36-1225-c1ae-2721b23b0299@huawei.com> Date: Thu, 15 Aug 2019 19:33:29 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <20190815093542.GA14313@quack2.suse.cz> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.244.145] X-CFilter-Loop: Reflected Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On 2019/8/15 17:35, Jan Kara Wrote: > On Thu 15-08-19 16:16:31, zhangyi (F) wrote: >> Remount process will release system zone which was allocated before if >> "noblock_validity" is specified. If we mount an ext4 file system to two >> mountpoints with default mount options, and then remount one of them >> with "noblock_validity", it may trigger a use after free problem when >> someone accessing the other one. >> >> # mount /dev/sda foo >> # mount /dev/sda bar >> >> User access mountpoint "foo" | Remount mountpoint "bar" >> | >> ext4_map_blocks() | ext4_remount() >> check_block_validity() | ext4_setup_system_zone() >> ext4_data_block_valid() | ext4_release_system_zone() >> | free system_blks rb nodes >> access system_blks rb nodes | >> trigger use after free | >> >> This problem can also be reproduced by one mountpint, At the same time, >> add_system_zone() can get called during remount as well so there can be >> racing ext4_data_block_valid() reading the rbtree at the same time. >> >> This patch add RCU to protect system zone from releasing or building >> when doing a remount which inverse current "noblock_validity" mount >> option. It assign the rbtree after the whole tree was complete and >> do actual freeing after rcu grace period, avoid any intermediate state. >> >> Signed-off-by: zhangyi (F) >> --- >> Changes since v3: >> - add comments before ext4_setup_system_zone() and >> ext4_release_system_zone() to explain why we need to serializes update >> sbi->system_blks pointer. >> - Fix block validity checking logic changes in v3. > > Thanks for the patch! The patch looks good. Just some language fixes in the > new comments below. You can add: > > Reviewed-by: Jan Kara > >> +/* >> + * Build system zone rbtree which is used for block validity checking. >> + * >> + * Note that system_blks pointer should be serializes updated at remount >> + * time even under sb->s_umount semaphore protection, due to it can be >> + * racing with ext4_data_block_valid() reading the system_blks rbtree at >> + * the same time. > > I'd rephrase this paragraph a bit to be easier to understand: > > The update of system_blks pointer in this function is protected by > sb->s_umount semaphore. However we have to be careful as we can be racing > with ext4_data_block_valid() calls reading system_blks rbtree protected > only by RCU. That's why we first build the rbtree and then swap it in place. > >> -/* Called when the filesystem is unmounted */ >> +/* >> + * Called when the filesystem is unmounted or when remounting it with >> + * noblock_validity specified. >> + * >> + * Note that system_blks pointer should be serializes updated and do >> + * the actual freeing after the RCU grace period at remount time even >> + * under sb->s_umount semaphore protection, due to it can be racing with >> + * ext4_data_block_valid() reading the system_blks rbtree at the same >> + * time. >> + */ > > Similarly here I'd phrase the last paragraph as: > > The update of system_blks pointer in this function is protected by > sb->s_umount semaphore. However we have to be careful as we can be racing > with ext4_data_block_valid() calls reading system_blks rbtree protected > only by RCU. So we first clear the system_blks pointer and then free the > rbtree only after RCU grace period expires. > Yes, it looks better, will do. Thanks, Yi.