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

[Laurent Vivier]

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.

I was adding a check before this function to be sure to not assert and
to report the error correctly to the QMP interface.

I think it is a step in the good direction.

Thanks,
Laurent