From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH-block] blk-cgroup: Use css_tryget() in blkcg_destroy_blkgs() Date: Mon, 28 Nov 2022 10:53:32 -0500 Message-ID: References: <20221128033057.1279383-1-longman@redhat.com> <427068db-6695-a1e2-4aa2-033220680eb9@kernel.dk> <786aacda-b25d-67f6-bad3-0030b0e2637e@kernel.dk> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1669650817; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/AJ0l8IgalwuHi6aQkbaTJH6CSV9/VHsYq9E6VyGE4U=; b=eSVRIpgMN4qnDtw6UyLGWp50zUGr667hJajlIZKEyCGApdax7Il/2A8tL1LDwduq8nWYv3 soV9XutBzerXB7jZ4YYVFbO1lWoF0iRSrHn+r0D30DhA9vV27kjPsH4XZ1RnJYIfVgWKV+ js1Kt7/VCa/jVTYNIMS1+Bu3edCZBnk= Content-Language: en-US In-Reply-To: <786aacda-b25d-67f6-bad3-0030b0e2637e-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org> List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Jens Axboe , Tejun Heo Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Ming Lei , Andy Shevchenko , Andrew Morton , =?UTF-8?Q?Michal_Koutn=c3=bd?= , Hillf Danton , Yi Zhang On 11/28/22 10:42, Jens Axboe wrote: > On 11/28/22 8:38?AM, Waiman Long wrote: >> On 11/28/22 09:14, Jens Axboe wrote: >>> On 11/27/22 8:30?PM, Waiman Long wrote: >>>> Commit 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction >>>> path") incorrectly assumes that css_get() will always succeed. That may >>>> not be true if there is no blkg associated with the blkcg. If css_get() >>>> fails, the subsequent css_put() call may lead to data corruption as >>>> was illustrated in a test system that it crashed on bootup when that >>>> commit was included. Also blkcg may be freed at any time leading to >>>> use-after-free. Fix it by using css_tryget() instead and bail out if >>>> the tryget fails. >>>> >>>> Fixes: 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction path") >>>> Reported-by: Yi Zhang >>>> Signed-off-by: Waiman Long >>>> --- >>>> ? block/blk-cgroup.c | 7 ++++++- >>>> ? 1 file changed, 6 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c >>>> index 57941d2a8ba3..74fefc8cbcdf 100644 >>>> --- a/block/blk-cgroup.c >>>> +++ b/block/blk-cgroup.c >>>> @@ -1088,7 +1088,12 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg) >>>> ? ????? might_sleep(); >>>> ? -??? css_get(&blkcg->css); >>>> +??? /* >>>> +???? * If css_tryget() fails, there is no blkg to destroy. >>>> +???? */ >>>> +??? if (!css_tryget(&blkcg->css)) >>>> +??????? return; >>>> + >>>> ????? spin_lock_irq(&blkcg->lock); >>>> ????? while (!hlist_empty(&blkcg->blkg_list)) { >>>> ????????? struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first, >>> This doesn't seem safe to me, but maybe I'm missing something. A tryget >>> operation can be fine if we're under RCU lock and the entity is freed >>> appropriately, but what makes it safe here? Could blkcg already be gone >>> at this point? >> The actual freeing of the blkcg structure is under RCU protection. So >> the structure won't be freed immediately even if css_tryget() fails. I >> suspect what Michal found may be the root cause of this problem. If >> so, this is an existing bug which gets exposed by my patch. > But what prevents it from going away here since you're not under RCU > lock for the tryget? Doesn't help that the freeing side is done in an > RCU safe manner, if the ref attempt is not. You are right. blkcg_destroy_blkgs() shouldn't be called with all the blkcg references gone. Will work on a revised patch. Cheers, Longman 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 5D21EC43217 for ; Mon, 28 Nov 2022 15:54:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232187AbiK1Pyj (ORCPT ); Mon, 28 Nov 2022 10:54:39 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60402 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232410AbiK1Pyi (ORCPT ); Mon, 28 Nov 2022 10:54:38 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4E2981DDF7 for ; Mon, 28 Nov 2022 07:53:42 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1669650821; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/AJ0l8IgalwuHi6aQkbaTJH6CSV9/VHsYq9E6VyGE4U=; b=fYjeZNQ6wo8HwCQHOfKjhstGjnrARMgzL6AHBJd8RinYrSvs0ZMwLBCs5smkpTGd+0kVoB TBgyBRmJQxXCa378bqRdEEwqSlA0LM20Dv+RomOT2GHfVTpjJBOhDZ2EaAD3T4dJU98Kqa mtFHqdp2ptZKufHMVN+ml/kY6/hwmmo= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-607-Uqa5zVnyMkuBLW5ut3eDSQ-1; Mon, 28 Nov 2022 10:53:36 -0500 X-MC-Unique: Uqa5zVnyMkuBLW5ut3eDSQ-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 626638027F5; Mon, 28 Nov 2022 15:53:35 +0000 (UTC) Received: from [10.22.10.34] (unknown [10.22.10.34]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8D3731415100; Mon, 28 Nov 2022 15:53:34 +0000 (UTC) Message-ID: Date: Mon, 28 Nov 2022 10:53:32 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH-block] blk-cgroup: Use css_tryget() in blkcg_destroy_blkgs() Content-Language: en-US To: Jens Axboe , Tejun Heo Cc: cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Ming Lei , Andy Shevchenko , Andrew Morton , =?UTF-8?Q?Michal_Koutn=c3=bd?= , Hillf Danton , Yi Zhang References: <20221128033057.1279383-1-longman@redhat.com> <427068db-6695-a1e2-4aa2-033220680eb9@kernel.dk> <786aacda-b25d-67f6-bad3-0030b0e2637e@kernel.dk> From: Waiman Long In-Reply-To: <786aacda-b25d-67f6-bad3-0030b0e2637e@kernel.dk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 11/28/22 10:42, Jens Axboe wrote: > On 11/28/22 8:38?AM, Waiman Long wrote: >> On 11/28/22 09:14, Jens Axboe wrote: >>> On 11/27/22 8:30?PM, Waiman Long wrote: >>>> Commit 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction >>>> path") incorrectly assumes that css_get() will always succeed. That may >>>> not be true if there is no blkg associated with the blkcg. If css_get() >>>> fails, the subsequent css_put() call may lead to data corruption as >>>> was illustrated in a test system that it crashed on bootup when that >>>> commit was included. Also blkcg may be freed at any time leading to >>>> use-after-free. Fix it by using css_tryget() instead and bail out if >>>> the tryget fails. >>>> >>>> Fixes: 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction path") >>>> Reported-by: Yi Zhang >>>> Signed-off-by: Waiman Long >>>> --- >>>> ? block/blk-cgroup.c | 7 ++++++- >>>> ? 1 file changed, 6 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c >>>> index 57941d2a8ba3..74fefc8cbcdf 100644 >>>> --- a/block/blk-cgroup.c >>>> +++ b/block/blk-cgroup.c >>>> @@ -1088,7 +1088,12 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg) >>>> ? ????? might_sleep(); >>>> ? -??? css_get(&blkcg->css); >>>> +??? /* >>>> +???? * If css_tryget() fails, there is no blkg to destroy. >>>> +???? */ >>>> +??? if (!css_tryget(&blkcg->css)) >>>> +??????? return; >>>> + >>>> ????? spin_lock_irq(&blkcg->lock); >>>> ????? while (!hlist_empty(&blkcg->blkg_list)) { >>>> ????????? struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first, >>> This doesn't seem safe to me, but maybe I'm missing something. A tryget >>> operation can be fine if we're under RCU lock and the entity is freed >>> appropriately, but what makes it safe here? Could blkcg already be gone >>> at this point? >> The actual freeing of the blkcg structure is under RCU protection. So >> the structure won't be freed immediately even if css_tryget() fails. I >> suspect what Michal found may be the root cause of this problem. If >> so, this is an existing bug which gets exposed by my patch. > But what prevents it from going away here since you're not under RCU > lock for the tryget? Doesn't help that the freeing side is done in an > RCU safe manner, if the ref attempt is not. You are right. blkcg_destroy_blkgs() shouldn't be called with all the blkcg references gone. Will work on a revised patch. Cheers, Longman