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=-7.4 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,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 714E4C433DB for ; Thu, 11 Feb 2021 11:29:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 355AC64DCF for ; Thu, 11 Feb 2021 11:29:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231604AbhBKL3G (ORCPT ); Thu, 11 Feb 2021 06:29:06 -0500 Received: from mx2.suse.de ([195.135.220.15]:40214 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231215AbhBKL0m (ORCPT ); Thu, 11 Feb 2021 06:26:42 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1613042754; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=1w2zYZzVu/kN/xdrmtBPuljrT1JGHIIdDgmFx1ldrRU=; b=E9iKLZQkZ3042WHpsi/v9r0SRvCwZ5P+U11IaeCy5pHu/RTe9Am6RbICJcl9gBEaDJTCKm mEaNQjOIgeHdXZtiGzpJC5HQPOIv1AMH+iuwCHthZKcTuH3sjYU8XJYVoAXKU5Z/Igub7f bOixfGTl8X+6W7qLR9cV0NVflFzd3+M= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id E4E78AD2B; Thu, 11 Feb 2021 11:25:53 +0000 (UTC) Subject: Re: [PATCH] btrfs: avoid double put of block group when emptying cluster To: dsterba@suse.cz, Josef Bacik , linux-btrfs@vger.kernel.org, kernel-team@fb.com, stable@vger.kernel.org References: <5ca694ff4f8cff4c0ef6896593a1f1d01fbe956d.1611610947.git.josef@toxicpanda.com> <20210210225014.GA1993@twin.jikos.cz> From: Nikolay Borisov Autocrypt: addr=nborisov@suse.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210210225014.GA1993@twin.jikos.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 11.02.21 г. 0:50 ч., David Sterba wrote: > On Tue, Jan 26, 2021 at 09:30:45AM -0500, Josef Bacik wrote: >> On 1/26/21 4:02 AM, Nikolay Borisov wrote: >>> On 25.01.21 г. 23:42 ч., Josef Bacik wrote: >>>> In __btrfs_return_cluster_to_free_space we will bail doing the cleanup >>>> of the cluster if the block group we passed in doesn't match the block >>>> group on the cluster. However we drop a reference to block_group, as >>>> the cluster holds a reference to the block group while it's attached to >>>> the cluster. If cluster->block_group != block_group however then this >>>> is an extra put, which means we'll go negative and free this block group >>>> down the line, leading to a UAF. >>> >>> Was this found by code inspection or did you hit in production. Also why >>> in btrfs_remove_free_space_cache just before >>> __btrfs_return_cluster_to_free_space there is: >>> >> >> It was found in production sort of halfway. I was doing something for WhatsApp >> and had to convert our block group reference counting to the refcount stuff so I >> could find where I made a mistake. Turns out this was where the problem was, my >> stuff had just made it way more likely to happen. I don't have the stack trace >> because this was like 6 months ago, I'm going through all my WhatsApp magic and >> getting them actually usable for upstream. >> >>> WARN_ON(cluster->block_group != block_group); >>> >>> IMO this patch should also remove the WARN_ON if it's a valid condition >>> to have the passed bg be different than the one in the cluster. Also >>> that WARN_ON is likely racy since it's done outside of cluster->lock. >>> >> >> Yup that's in a follow up thing, I wanted to get the actual fix out before I got >> distracted by my mountain of meetings this week. Thanks, > > Removing the WARN_ON in a separate patch sounds ok to me, this patch > clearly fixes the refcounting bug, the warning condition is the same but > would need a different reasoning. > > Nikolay, if you're ok with current patch version let me know if you want > a rev-by added. > Codewise I'm fine with it. However just had another read of the commit message and I think it could be rewritten to be somewhat simpler: It's wrong calling btrfs_put_block_group in __btrfs_return_cluster_to_free_space if the block group passed is different than the block group the cluster represents. As this means the cluster doesn't have a reference to the passed block group. This results in double put and an UAF. What prompted me is that the 2nd and 3rd sentences read somewhat awkward due to starting with 'However'