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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 43903C433E1 for ; Thu, 4 Jun 2020 06:10:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 210C52072E for ; Thu, 4 Jun 2020 06:10:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="MCijP08i" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726987AbgFDGKm (ORCPT ); Thu, 4 Jun 2020 02:10:42 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:24224 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726809AbgFDGKl (ORCPT ); Thu, 4 Jun 2020 02:10:41 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1591251039; 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=spuln8OM2+pVMSm6QcNLxZDaud+7HSx0EW1bt/tNWEk=; b=MCijP08ima0bYx5d+bBh2AR3YSjBepOxdOX9B5wCWUscALsBlQ1T38vJhbCYWA4IKjdil0 TMrehH75DzWUnoCVyIFfvLogLodL6znq8+FCX5OgGmTYQK/XGIC6KFqgwZf8WE/X7BV6ly Ol7kYt2HfoGPWYfAIXO9tFoZbncm4qY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-346-U7aO9fZrPSCuhDk-WhhKeA-1; Thu, 04 Jun 2020 02:10:38 -0400 X-MC-Unique: U7aO9fZrPSCuhDk-WhhKeA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3AA0B108BD0D; Thu, 4 Jun 2020 06:10:37 +0000 (UTC) Received: from [10.72.13.104] (ovpn-13-104.pek2.redhat.com [10.72.13.104]) by smtp.corp.redhat.com (Postfix) with ESMTP id C5D234DA21; Thu, 4 Jun 2020 06:10:28 +0000 (UTC) Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok() To: Al Viro , "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org References: <20200602084257.134555-1-mst@redhat.com> <20200603014815.GR23230@ZenIV.linux.org.uk> <20200603011810-mutt-send-email-mst@kernel.org> <20200603165205.GU23230@ZenIV.linux.org.uk> From: Jason Wang Message-ID: Date: Thu, 4 Jun 2020 14:10:27 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200603165205.GU23230@ZenIV.linux.org.uk> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020/6/4 上午12:52, Al Viro wrote: > On Wed, Jun 03, 2020 at 01:29:00AM -0400, Michael S. Tsirkin wrote: >> On Wed, Jun 03, 2020 at 02:48:15AM +0100, Al Viro wrote: >>> On Tue, Jun 02, 2020 at 04:45:05AM -0400, Michael S. Tsirkin wrote: >>>> So vhost needs to poke at userspace *a lot* in a quick succession. It >>>> is thus benefitial to enable userspace access, do our thing, then >>>> disable. Except access_ok has already been pre-validated with all the >>>> relevant nospec checks, so we don't need that. Add an API to allow >>>> userspace access after access_ok and barrier_nospec are done. >>> BTW, what are you going to do about vq->iotlb != NULL case? Because >>> you sure as hell do *NOT* want e.g. translate_desc() under STAC. >>> Disable it around the calls of translate_desc()? >>> >>> How widely do you hope to stretch the user_access areas, anyway? >> So ATM I'm looking at adding support for the packed ring format. >> That does something like: >> >> get_user(flags, desc->flags) >> smp_rmb() >> if (flags & VALID) >> copy_from_user(&adesc, desc, sizeof adesc); >> >> this would be a good candidate I think. > Perhaps, once we get stac/clac out of raw_copy_from_user() (coming cycle, > probably). BTW, how large is the structure and how is it aligned? Each descriptor is 16 bytes, and 16 bytes aligned. > >>> BTW, speaking of possible annotations: looks like there's a large >>> subset of call graph that can be reached only from vhost_worker() >>> or from several ioctls, with all uaccess limited to that subgraph >>> (thankfully). Having that explicitly marked might be a good idea... >> Sure. What's a good way to do that though? Any examples to follow? >> Or do you mean code comments? > Not sure... FWIW, the part of call graph from "known to be only > used by vhost_worker" (->handle_kick/vhost_work_init callback/ > vhost_poll_init callback) and "part of ->ioctl()" to actual uaccess > primitives is fairly large - the longest chain is > handle_tx_net -> > handle_tx -> > handle_tx_zerocopy -> > get_tx_bufs -> > vhost_net_tx_get_vq_desc -> > vhost_tx_batch -> > vhost_net_signal_used -> > vhost_add_used_and_signal_n -> > vhost_signal -> > vhost_notify -> > vhost_get_avail_flags -> > vhost_get_avail -> > vhost_get_user -> > __get_user() > i.e. 14 levels deep and the graph doesn't factorize well... > > Something along the lines of "all callers of thus annotated function > must be annotated the same way themselves, any implicit conversion > of pointers to such functions to anything other than boolean yields > a warning, explicit cast is allowed only with __force", perhaps? > Then slap such annotations on vhost_{get,put,copy_to,copy_from}_user(), > on ->handle_kick(), a force-cast in the only caller of ->handle_kick() > and force-casts in the 3 callers in ->ioctl(). > > And propagate the annotations until the warnings stop, basically... > > Shouldn't be terribly hard to teach sparse that kind of stuff and it > might be useful elsewhere. It would act as a qualifier on function > pointers, with syntax ultimately expanding to __attribute__((something)). > I'll need to refresh my memories of the parser, but IIRC that shouldn't > require serious modifications. Most of the work would be in > evaluate_call(), just before calling evaluate_symbol_call()... > I'll look into that; not right now, though. > > BTW, __vhost_get_user() might be better off expanded in both callers - > that would get their structure similar to vhost_copy_{to,from}_user(), > especially if you expand __vhost_get_user_slow() as well. > > Not sure I understand what's going with ->meta_iotlb[] - what are the > lifetime rules for struct vhost_iotlb_map It used to cache the translation for virtqueue address which. Vhost will try to get those addresses from IOTLB and store them in meta_iotlb, and it will be invalidated when userspace update or invalidate a new mapping. > and what prevents the pointers > from going stale? > > The vq->mutex is used to synchronize between the invalidation and vhost workers. Thanks