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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham 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 06732C7618B for ; Tue, 23 Jul 2019 13:16:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D4CF7218BE for ; Tue, 23 Jul 2019 13:16:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390115AbfGWNQ0 (ORCPT ); Tue, 23 Jul 2019 09:16:26 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52092 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727311AbfGWNQ0 (ORCPT ); Tue, 23 Jul 2019 09:16:26 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id F07623082AF2; Tue, 23 Jul 2019 13:16:25 +0000 (UTC) Received: from [10.72.12.26] (ovpn-12-26.pek2.redhat.com [10.72.12.26]) by smtp.corp.redhat.com (Postfix) with ESMTP id E01A35D9C5; Tue, 23 Jul 2019 13:16:16 +0000 (UTC) Subject: Re: [PATCH 6/6] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps() To: "Michael S. Tsirkin" Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20190723075718.6275-1-jasowang@redhat.com> <20190723075718.6275-7-jasowang@redhat.com> <20190723041144-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Tue, 23 Jul 2019 21:16:20 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190723041144-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Tue, 23 Jul 2019 13:16:26 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/7/23 下午5:16, Michael S. Tsirkin wrote: > On Tue, Jul 23, 2019 at 03:57:18AM -0400, Jason Wang wrote: >> There's no need for RCU synchronization in vhost_uninit_vq_maps() >> since we've already serialized with readers (memory accessors). This >> also avoid the possible userspace DOS through ioctl() because of the >> possible high latency caused by synchronize_rcu(). >> >> Reported-by: Michael S. Tsirkin >> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address") >> Signed-off-by: Jason Wang > I agree synchronize_rcu in both mmu notifiers and ioctl > is a problem we must fix. > >> --- >> drivers/vhost/vhost.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index 5b8821d00fe4..a17df1f4069a 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -334,7 +334,9 @@ static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq) >> } >> spin_unlock(&vq->mmu_lock); >> >> - synchronize_rcu(); >> + /* No need for synchronize_rcu() or kfree_rcu() since we are >> + * serialized with memory accessors (e.g vq mutex held). >> + */ >> >> for (i = 0; i < VHOST_NUM_ADDRS; i++) >> if (map[i]) >> -- >> 2.18.1 > .. however we can not RCU with no synchronization in sight. > Sometimes there are hacks like using a lock/unlock > pair instead of sync, but here no one bothers. > > specifically notifiers call reset vq maps which calls > uninit vq maps which is not under any lock. Notifier did this:         if (map) {                 if (uaddr->write) {                         for (i = 0; i < map->npages; i++) set_page_dirty(map->pages[i]); }                 rcu_assign_pointer(vq->maps[index], NULL); } spin_unlock(&vq->mmu_lock);         if (map) { synchronize_rcu(); vhost_map_unprefetch(map);         } So it indeed have a synchronize_rcu() there. Thanks > > You will get use after free when map is then accessed. > > If you always have a lock then just take that lock > and no need for RCU. >