Re: [PATCH] net: tap: check if the file descriptor is valid before using it

[Daniel P . Berrangé]

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
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