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 :|
