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Tue, 30 Jun 2020 09:31:51 +0000 (UTC) Date: Tue, 30 Jun 2020 10:31:48 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Jason Wang Subject: Re: [PATCH] net: tap: check if the file descriptor is valid before using it Message-ID: <20200630093148.GF1370404@redhat.com> 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> MIME-Version: 1.0 In-Reply-To: <20200630092318.GE1370404@redhat.com> User-Agent: Mutt/1.14.3 (2020-06-14) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 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=207.211.31.120; envelope-from=berrange@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/30 00:34:33 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: Laurent Vivier , Stefan Weil , 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 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 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 :|