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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 9F6D1C433EF for ; Fri, 10 Dec 2021 14:02:15 +0000 (UTC) Received: from localhost ([::1]:52820 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mvgTW-0005gL-5B for qemu-devel@archiver.kernel.org; Fri, 10 Dec 2021 09:02:14 -0500 Received: from eggs.gnu.org ([209.51.188.92]:40392) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mvgSV-0004rf-Tj for qemu-devel@nongnu.org; Fri, 10 Dec 2021 09:01:11 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:30375) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mvgSS-0004n5-Cx for qemu-devel@nongnu.org; Fri, 10 Dec 2021 09:01:10 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1639144867; 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: in-reply-to:in-reply-to:references:references; bh=xMeDsmAgu9Q+F0/ufoihzFGJ1mStNM8UQEidhO8LsYs=; b=SLPN4ttaFFQeSdEHtiul76FUNMSPxuALuQf6gwXdpfgCOXnnRljHMwv8X/5LoPjmswazEA TOQu0VmVPrFijK9ZeyB3SpPM/cbw4ofi3lXhksrC0UndpoHd60Cwb79eY8YgIzrPHr6W5K Ky+Vc0iH+inI+zovcMMZmFA4BNv+TvA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-294-8Ea5H5n9OAWRTsM9R-o08g-1; Fri, 10 Dec 2021 09:01:04 -0500 X-MC-Unique: 8Ea5H5n9OAWRTsM9R-o08g-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 451B210168D0; Fri, 10 Dec 2021 14:01:03 +0000 (UTC) Received: from redhat.com (unknown [10.39.192.255]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D865960C9F; Fri, 10 Dec 2021 14:00:39 +0000 (UTC) Date: Fri, 10 Dec 2021 15:00:38 +0100 From: Kevin Wolf To: Stefan Hajnoczi Subject: Re: [RFC] block-backend: prevent dangling BDS pointer in blk_drain() Message-ID: References: <20211209142304.381253-1-stefanha@redhat.com> MIME-Version: 1.0 In-Reply-To: <20211209142304.381253-1-stefanha@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.619, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Hanna Reitz , Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 09.12.2021 um 15:23 hat Stefan Hajnoczi geschrieben: > The BlockBackend root child can change during bdrv_drained_begin() when > aio_poll() is invoked. In fact the BlockDriverState can reach refcnt 0 > and blk_drain() is left with a dangling BDS pointer. > > One example is scsi_device_purge_requests(), which calls blk_drain() to > wait for in-flight requests to cancel. If the backup blockjob is active, > then the BlockBackend root child is a temporary filter BDS owned by the > blockjob. The blockjob can complete during bdrv_drained_begin() and the > last reference to the BDS is released when the temporary filter node is > removed. This results in a use-after-free when blk_drain() calls > bdrv_drained_end(bs) on the dangling pointer. > > The general problem is that a function and its callers must not assume > that bs is still valid across aio_poll(). Explicitly hold a reference to > bs in blk_drain() to avoid the dangling pointer. > > Signed-off-by: Stefan Hajnoczi > --- > I found that BDS nodes are sometimes deleted with bs->quiesce_counter > > 0 (at least when running "make check"), so it is currently not possible > to put the bdrv_ref/unref() calls in bdrv_do_drained_begin() and > bdrv_do_drained_end() because they will be unbalanced. That would have > been a more general solution than only fixing blk_drain(). They are not supposed to end up unbalanced because detaching a child calls bdrv_unapply_subtree_drain(). In fact, I think test-bdrv-drain tests a few scenarios like this. Do have more details about the case that failed for you? Kevin