On Tue, Jun 30, 2020 at 02:00:06PM +0200, Laurent Vivier wrote: > On 30/06/2020 13:03, Daniel P. Berrangé wrote: > > 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. > > The problem here is the very first operation of net_init_tap() is a > qemu_set_nonblock() that has an assert() and crashes QEMU. > > It's why I was only checking for the validity of the file descriptor, > not if it is a tap device or not.
Yep, checking that it is really a FD is sufficient to avoid the assert in nonblock. As for whether it is really a tap device, I think we just need to improve error reporting of the functions that come later, instead of doing a literal "is it a tap" check. That's what I'd tried in my old patch from a few years back https://patchwork.kernel.org/patch/10029443/ I can't remember why we didn't merge this back then 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 :|
