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 A613CC433FE for ; Mon, 28 Nov 2022 15:39:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230509AbiK1Pj4 (ORCPT ); Mon, 28 Nov 2022 10:39:56 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48324 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231299AbiK1Pjx (ORCPT ); Mon, 28 Nov 2022 10:39:53 -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 ADABB1F627 for ; Mon, 28 Nov 2022 07:38:53 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1669649932; 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=4r34gL/zLUGsXD6KoTRJJSoISJJekVdoxgz5n6tlG2k=; b=G5/t9+EYBksbzkLhMS3wm2ua+N9vqJbzfZaggpHpYdnTGfnFrCoVppwUa2epNfxEYqd4e4 +4tazdLmk9SpK873rTPAnxKPfLeeYYl+ZAXgOEX3EA1MMjz4KrQG9j3jTRBc+amAxtFQdK JMA7lWJqHumZO25WcVzc0LlPwK8VLT4= 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-159-VCRQlnyDOiGbQ_EdikapSQ-1; Mon, 28 Nov 2022 10:38:48 -0500 X-MC-Unique: VCRQlnyDOiGbQ_EdikapSQ-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7368F802C17; Mon, 28 Nov 2022 15:38:47 +0000 (UTC) Received: from [10.22.10.34] (unknown [10.22.10.34]) by smtp.corp.redhat.com (Postfix) with ESMTP id B1EB6492B05; Mon, 28 Nov 2022 15:38:46 +0000 (UTC) Message-ID: Date: Mon, 28 Nov 2022 10:38:44 -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> From: Waiman Long In-Reply-To: <427068db-6695-a1e2-4aa2-033220680eb9@kernel.dk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. Cheers, Longman