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Tue, 30 Jun 2020 10:57:55 +0000 (UTC) Subject: Re: [PATCH] net: tap: check if the file descriptor is valid before using it To: Laurent Vivier , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20200624190009.300069-1-lvivier@redhat.com> <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> From: Jason Wang Message-ID: <466e86a9-eb74-c147-2c3c-cb546176b944@redhat.com> Date: Tue, 30 Jun 2020 18:57:54 +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: <7a110325-0123-53da-604d-8a9374903782@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jasowang@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=207.211.31.81; envelope-from=jasowang@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 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: , Cc: Stefan Weil , Paolo Bonzini , qemu-devel@nongnu.org, Markus Armbruster , Gerd Hoffmann Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2020/6/30 下午6:35, 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. Right. > > 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, Yes. > but how to do that with tuntap > that does not create a /dev/tapY device? I think there's no specific reason, it's just because it was wrote like that since the first version which is about 20 years ago. Thanks > > Thanks, > Laurent > >