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

[Jason Wang <jasowang@redhat.com>]

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.

Thanks


>
> Thanks,
> Laurent
>
>