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 A872BC43219 for ; Wed, 15 Dec 2021 16:11:54 +0000 (UTC) Received: from localhost ([::1]:34266 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mxWGu-0001Yh-RE for qemu-devel@archiver.kernel.org; Wed, 15 Dec 2021 10:32:48 -0500 Received: from eggs.gnu.org ([209.51.188.92]:41206) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mxWFt-0008UG-Er for qemu-devel@nongnu.org; Wed, 15 Dec 2021 10:31:46 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:50349) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mxWFq-0007km-IJ for qemu-devel@nongnu.org; Wed, 15 Dec 2021 10:31:44 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1639582301; 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=oqlz1tiNpEviH2PMIr4dQt/gv+P/L0F7L7mMSyJHUjA=; b=Vfi6MFpyP/qYjx1g6KqTtMYekonbbNPyR3iPOL3bO3mh/wFF7qdEOHExoE8jsuqLPWESg1 vskDb02fYoUU6+YsIo0OjT1m2zaPhkyiN6Um2G8ASbbMIivxNHByRcUAR9XvlmKxfCIHgR qN0NerbKiFQZtrD09Q9OQaIGztHdAEo= 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-74-NudRc8wiNSiPx6hVuEJF4Q-1; Wed, 15 Dec 2021 10:31:34 -0500 X-MC-Unique: NudRc8wiNSiPx6hVuEJF4Q-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7A4A618C89E8; Wed, 15 Dec 2021 15:31:30 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.249]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D56805BE00; Wed, 15 Dec 2021 15:31:27 +0000 (UTC) Date: Wed, 15 Dec 2021 16:31:26 +0100 From: Kevin Wolf To: Stefan Hajnoczi Subject: Re: [PATCH v2] block-backend: prevent dangling BDS pointers across aio_poll() Message-ID: References: <20211214143542.14758-1-stefanha@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="1xBMTRFBH24oiMlL" 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: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.719, 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=ham 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, qemu-stable@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --1xBMTRFBH24oiMlL Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 15.12.2021 um 12:28 hat Stefan Hajnoczi geschrieben: > On Tue, Dec 14, 2021 at 03:59:49PM +0100, Kevin Wolf wrote: > > Am 14.12.2021 um 15:35 hat Stefan Hajnoczi geschrieben: > > > The BlockBackend root child can change when aio_poll() is invoked. Th= is > > > happens when a temporary filter node is removed upon blockjob > > > completion, for example. > > >=20 > > > Functions in block/block-backend.c must be aware of this when using a > > > blk_bs() pointer across aio_poll() because the BlockDriverState refcn= t > > > may reach 0, resulting in a stale pointer. > > >=20 > > > One example is scsi_device_purge_requests(), which calls blk_drain() = to > > > wait for in-flight requests to cancel. If the backup blockjob is acti= ve, > > > then the BlockBackend root child is a temporary filter BDS owned by t= he > > > blockjob. The blockjob can complete during bdrv_drained_begin() and t= he > > > 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. > > >=20 > > > Explicitly hold a reference to bs across block APIs that invoke > > > aio_poll(). > > >=20 > > > Signed-off-by: Stefan Hajnoczi > > > --- > > > v2: > > > - Audit block/block-backend.c and fix additional cases > > > --- > > > block/block-backend.c | 11 +++++++++++ > > > 1 file changed, 11 insertions(+) > > >=20 > > > diff --git a/block/block-backend.c b/block/block-backend.c > > > index 12ef80ea17..a40ad7fa92 100644 > > > --- a/block/block-backend.c > > > +++ b/block/block-backend.c > > > @@ -828,10 +828,12 @@ void blk_remove_bs(BlockBackend *blk) > > > notifier_list_notify(&blk->remove_bs_notifiers, blk); > > > if (tgm->throttle_state) { > > > bs =3D blk_bs(blk); > > > + bdrv_ref(bs); > > > bdrv_drained_begin(bs); > > > throttle_group_detach_aio_context(tgm); > > > throttle_group_attach_aio_context(tgm, qemu_get_aio_context(= )); > > > bdrv_drained_end(bs); > > > + bdrv_unref(bs); > > > } > > > =20 > > > blk_update_root_state(blk); > >=20 > > This hunk is unnecessary, we still hold a reference that is only given > > up a few lines down with bdrv_root_unref_child(root). >=20 > That's not the only place where the reference can be dropped: > bdrv_drop_filter() removes the filter node from the graph. >=20 > Here is a case where it happens: block/backup.c:backup_clean() -> > bdrv_cbw_drop() -> bdrv_drop_filter() -> bdrv_replace_node_common() -> > bdrv_replace_child_commit(). After we reach this bdrv_unref() is called > a few times and all references are dropped because the node is no longer > in the graph. >=20 > This happens during blk_remove_bs() -> bdrv_drained_begin(), so the bs > pointer in the above hunk can be stale. Is the scenario that blk->root doesn't go away, but the node it references changes? So the unref below will be for a different node than we're draining here? If so, let's add a comment that blk_bs(blk) can change after the drain, and maybe move the BlockDriverState *bs declaration inside the if block because the variable is invalid after it anyway. Can it also happen that instead of removing a node from the chain, a new one is inserted and we actually end up having drained the wrong node before switching the context for tgm? 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