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Tue, 30 Jun 2020 09:21:51 +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> From: Jason Wang Message-ID: Date: Tue, 30 Jun 2020 17:21:49 +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: Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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=205.139.110.120; envelope-from=jasowang@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/30 03:55:26 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: qemu-devel@nongnu.org, Stefan Weil , Gerd Hoffmann , Markus Armbruster , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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. Thanks > > Thanks, > Laurent > >