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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, 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 3F23DC31E40 for ; Thu, 15 Aug 2019 09:35:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1FE69208C2 for ; Thu, 15 Aug 2019 09:35:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731179AbfHOJfo (ORCPT ); Thu, 15 Aug 2019 05:35:44 -0400 Received: from mx2.suse.de ([195.135.220.15]:59004 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730212AbfHOJfo (ORCPT ); Thu, 15 Aug 2019 05:35:44 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 8196DB12E; Thu, 15 Aug 2019 09:35:42 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 172DE1E4200; Thu, 15 Aug 2019 11:35:42 +0200 (CEST) Date: Thu, 15 Aug 2019 11:35:42 +0200 From: Jan Kara To: "zhangyi (F)" Cc: linux-ext4@vger.kernel.org, tytso@mit.edu, jack@suse.cz, adilger.kernel@dilger.ca Subject: Re: [PATCH v4] ext4: fix potential use after free in system zone via remount with noblock_validity Message-ID: <20190815093542.GA14313@quack2.suse.cz> References: <20190815081631.19437-1-yi.zhang@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190815081631.19437-1-yi.zhang@huawei.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org 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. Honza -- Jan Kara SUSE Labs, CR