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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 4A45AC433E0 for ; Tue, 30 Jun 2020 11:04:28 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 11A202067D for ; Tue, 30 Jun 2020 11:04:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="bWRZjEaM" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 11A202067D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:52438 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jqE3v-0006cV-Bz for qemu-devel@archiver.kernel.org; Tue, 30 Jun 2020 07:04:27 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:46548) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jqE2z-00062M-9j for qemu-devel@nongnu.org; Tue, 30 Jun 2020 07:03:29 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:22193 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jqE2x-0006Kb-1F for qemu-devel@nongnu.org; Tue, 30 Jun 2020 07:03:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1593515006; h=from:from:reply-to: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=Zcj9qsdWh1ZJWwvONxX6MREQYwmxR+n7OlNW327iP8I=; b=bWRZjEaMXjoxOZGJGUBnMJ6sH9LWwRcq/U83971z/Ou8p1bfc7ZCuAIGRH6LCMFwrvlXaD cpAWxTBE+aVLYpACHW8/SOK91PlU5OxYSsObFdAnnngKCmyHqVzDVuNPqF2OyotDYKhjWB mdvG+nvbuTTYtdnpGIajFf1sLQhGqPc= 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-471-ZSo0Xj2tMAKmAeRCu2cKNg-1; Tue, 30 Jun 2020 07:03:19 -0400 X-MC-Unique: ZSo0Xj2tMAKmAeRCu2cKNg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C23ED10059A1; Tue, 30 Jun 2020 11:03:18 +0000 (UTC) Received: from redhat.com (unknown [10.36.110.30]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 635AE1002388; Tue, 30 Jun 2020 11:03:13 +0000 (UTC) Date: Tue, 30 Jun 2020 12:03:10 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Laurent Vivier Subject: Re: [PATCH] net: tap: check if the file descriptor is valid before using it Message-ID: <20200630110310.GG1370404@redhat.com> References: <20200625084835.GB1009994@redhat.com> <541b40c5-ee72-d37d-1c30-664775812d1b@redhat.com> <929203d2-20d2-7caf-e487-6bfe5b851974@redhat.com> <20200630092318.GE1370404@redhat.com> <20200630093148.GF1370404@redhat.com> <247f4aa8-1846-c5ec-4fe3-1d344979ad52@redhat.com> <7a110325-0123-53da-604d-8a9374903782@redhat.com> MIME-Version: 1.0 In-Reply-To: <7a110325-0123-53da-604d-8a9374903782@redhat.com> User-Agent: Mutt/1.14.3 (2020-06-14) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.81; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/30 02:00:02 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Stefan Weil , Jason Wang , qemu-devel@nongnu.org, Markus Armbruster , Gerd Hoffmann , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Jun 30, 2020 at 12:35:46PM +0200, Laurent Vivier wrote: > On 30/06/2020 12:03, Jason Wang wrote: > > > > On 2020/6/30 下午5:45, Laurent Vivier wrote: > >> On 30/06/2020 11:31, Daniel P. Berrangé wrote: > >>> On Tue, Jun 30, 2020 at 10:23:18AM +0100, Daniel P. Berrangé wrote: > >>>> On Tue, Jun 30, 2020 at 05:21:49PM +0800, Jason Wang wrote: > >>>>> On 2020/6/30 上午3:30, Laurent Vivier wrote: > >>>>>> On 28/06/2020 08:31, Jason Wang wrote: > >>>>>>> On 2020/6/25 下午7:56, Laurent Vivier wrote: > >>>>>>>> On 25/06/2020 10:48, Daniel P. Berrangé wrote: > >>>>>>>>> On Wed, Jun 24, 2020 at 09:00:09PM +0200, Laurent Vivier wrote: > >>>>>>>>>> qemu_set_nonblock() checks that the file descriptor can be > >>>>>>>>>> used and, if > >>>>>>>>>> not, crashes QEMU. An assert() is used for that. The use of > >>>>>>>>>> assert() is > >>>>>>>>>> used to detect programming error and the coredump will allow > >>>>>>>>>> to debug > >>>>>>>>>> the problem. > >>>>>>>>>> > >>>>>>>>>> But in the case of the tap device, this assert() can be > >>>>>>>>>> triggered by > >>>>>>>>>> a misconfiguration by the user. At startup, it's not a real > >>>>>>>>>> problem, > >>>>>>>>>> but it > >>>>>>>>>> can also happen during the hot-plug of a new device, and here > >>>>>>>>>> it's a > >>>>>>>>>> problem because we can crash a perfectly healthy system. > >>>>>>>>> If the user/mgmt app is not correctly passing FDs, then there's > >>>>>>>>> a whole > >>>>>>>>> pile of bad stuff that can happen. Checking whether the FD is > >>>>>>>>> valid is > >>>>>>>>> only going to catch a small subset. eg consider if fd=9 refers > >>>>>>>>> to the > >>>>>>>>> FD that is associated with the root disk QEMU has open. We'll > >>>>>>>>> fail to > >>>>>>>>> setup the TAP device and close this FD, breaking the healthy > >>>>>>>>> system > >>>>>>>>> again. > >>>>>>>>> > >>>>>>>>> I'm not saying we can't check if the FD is valid, but lets be > >>>>>>>>> clear that > >>>>>>>>> this is not offering very much protection against a broken mgmt > >>>>>>>>> apps > >>>>>>>>> passing bad FDs. > >>>>>>>>> > >>>>>>>> I agree with you, but my only goal here is to avoid the crash in > >>>>>>>> this > >>>>>>>> particular case. > >>>>>>>> > >>>>>>>> The punishment should fit the crime. > >>>>>>>> > >>>>>>>> The user can think the netdev_del doesn't close the fd, and he > >>>>>>>> can try > >>>>>>>> to reuse it. Sending back an error is better than crashing his > >>>>>>>> system. > >>>>>>>> After that, if the system crashes, it will be for the good > >>>>>>>> reasons, not > >>>>>>>> because of an assert. > >>>>>>> Yes. And on top of this we may try to validate the TAP via st_dev > >>>>>>> through fstat[1]. > >>>>>> I agree, but the problem I have is to know which major(st_dev) we can > >>>>>> allow to use. > >>>>>> > >>>>>> Do we allow only macvtap major number? > >>>>> > >>>>> Macvtap and tuntap. > >>>>> > >>>>> > >>>>>> How to know the macvtap major number at user level? > >>>>>> [it is allocated dynamically: do we need to parse /proc/devices?] > >>>>> > >>>>> I think we can get them through fstat for /dev/net/tun and > >>>>> /dev/macvtapX. > >>>> Don't assume QEMU has any permission to access to these device nodes, > >>>> only the pre-opened FDs it is given by libvirt. > >>> Actually permissions are the least of the problem - the device nodes > >>> won't even exist, because QEMU's almost certainly running in a private > >>> mount namespace with a minimal /dev populated > >>> > >> I'm working on a solution using /proc/devices. > > > > > > Similar issue with /dev. There's no guarantee that qemu can access > > /proc/devices or it may not exist (CONFIG_PROCFS). > > There is a lot of things that will not work without /proc (several tools > rely on /proc, like ps, top, lsof, mount, ...). Some information are > only available from /proc, and if /proc is there, I think /proc/devices > is always readable by everyone. Moreover /proc is already used by qemu > in several places. > > It can also a best effort check. > > The problem with fstat() on /dev files is to guess the /dev/macvtapX as > X varies (the same with /dev/tapY).. > > > > >> macvtap has its own major number, but tuntap use "misc" (10) major > >> number. > > Another question: it is possible to use the "fd=" parameter with macvtap > as macvtap creates a /dev/tapY device, but how to do that with tuntap > that does not create a /dev/tapY device? I think we should step back and ask why we need to check this at all. IMHO, if the passed-in FD works with the syscalls that tap-linux.c is executing, then that shows the FD is suitable for QEMU. The problem is that many of the tap APIs don't use "Error **errp" parameters to report errors, so we can't catch the failures. IOW, instead of checking the FD major/minor number, we should make the existing code be better at reporting errors, so they can be fed back to the QMP console gracefully. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|